Chan Buddhism flourished during the Tang period and reached its peak in the Song era and became the dominant faction in the history of Chinese Buddhism. During its ″golden age,″ Chan actually went beyond the walls of the temples and permeated the daily life of the general public. In the process of localization of Chan thoughts, the most intriguing was koan — a crucial form in Chan language. Koan played an important role by providing a basis for wider recognition of legitimate transmission for Chan to thrive. The essence and beauty of the koan is to be rationally unresolvable and thus to point to what is arational, i.e. to abandon the rational thought structures and step beyond the usual state of consciousness. Chan is well known for ″a special transmission outside the teachings that does not rely on words and letters.″ Chan teachers recognized the limitation of language to describe the genuine enlightenment and emancipation. They tended to dispense their wisdom in an array of peculiar or unconventional ways, which included shouts and beatings that were meted out to their eager disciples. In order to achieve the ultimate goal of awakening, disciples must first encounter the impasse triggered by koan and transcend it. As a matter of fact, Chan masters could not totally reject the language, but had to use language creatively or play the language game by harnessing the innate ambiguity in natural language. A wide variety of legendary anecdotes about generations of famous Chan masters were recorded, who could ″point directly″ to awakening.  The present study presents an individualistic attempt to investigate the koan — the questions and answers between Chan masters and their disciples, from a pragmatic perspective, especially based on Cooperative Principle (CP) and conversational implicature. Examples from the koan case collections such as Zutang Ji and Jingde Chuandeng Lu are analyzed under the framework of CP and its four maxims. The discovery is that koan language is replete with the breach and violation of the four maxims in CP, i.e. Maxims of Quantity, Quality, Relation, and Manner. Judged from the Maxim of Quantity, Chan masters used the language that was highly elliptical and economical. Their answers were so concise that they might consist of single words or wordless ones. It is also not uncommon to find paradoxical speeches and non sequiturs, a mere violation to the Maxim of Quality. The Maxim of Relation is breached when masters deliberately used questions to answer questions, often leading to irrelevant and incoherent dialogues. Finally, obscure and unclear ways of speaking are also preferred by the masters. That proves to be a violation of the Maxim of Manner whose requirement is to speak clearly and to the point. Chan masters even taught in bizarre non-linguistic ways, such as ″shouting and hitting,″ not to mention the sacrilegious acts like burning the sutras or insulting its founding patriarch.  Koan and its whole process of communication can be better understood only through the lens of the context. The success of the spirited exchange relies on the three ingredients in the context: content, participants and the setting. Chan insights are based on every day facts and matters and with the personal experience as a starting point. The creativity and imagination of the Chan masters evoke an awakening by transcending ordinary language through powerfully direct means, then and there. It is a response to the limitation and divergence of the language and thus possesses performative force. Under the broader social, religious and cultural backdrop, the exchange of koan is revealed as formal non-cooperation to reach substantial cooperation — the religious goal-sharing of enlightenment and absolute liberation. Centuries of vitality and dynamism of the Chan practice were witnessed in the Tang-Song periods.  However, a further reflection from a historical point of view suggests that the over-dependence of the language and stylistic pursuit of koan may in turn refrain its blooming and eventually lead to the decline and deviation from its religious origin. During the Song era, the elaborate style was much utilized to meet the literary tastes of the Song literati, who were major patrons of Chan monasteries. The historical personage of the earlier eminent masters became approachable only via thick layers of meandering poetic and prose commentary or interpretation. As a result, Chan could not escape its doomed fate of degeneration into a vehicle of emotional cure and spiritual cultivation rather than the seeking of nirvana.