Abstract: Brittleness is one key parameter during drilling and fracturing in shale formations, however, influencing mechanism of soaking by drilling & completion fluid on shale brittleness has not been focused. In this paper, triaxial mechanical tests of shales soaked by fluid were performed; and optimized brittleness evaluation model was employed to evaluate brittleness of shale. Results indicate that the brittleness of Yanchang shale is larger than that of Longmaxi shale, and two kinds of drilling & completion fluid can decrease shale brittleness, in which oil-based drilling & completion fluid reduces shale brittleness more evidently. The reduction of brittleness Longmaxi shale is greater than that of Yanchang shale. It is demonstrated that five factors can weaken the brittleness of shale as follows: (1) Higher strength of bedding plane can lead to lower brittleness; (2) Spontaneous imbibition causes high pore pressure and great stress intensity factor of fractures; (3) Alkali erosion can cause mineral grains crack and dissolution pore; (4) Hydrate swelling of shale results in swelling stress; (5) Lubrication of drilling & completion filtrate results in lower friction coe cient. Yanchang shale has lower bedding plane strength. So the brittleness of Yanchang shale is larger than that of Longmaxi shale; Oil-based drilling & completion fluid has greater imbibition amount, higher pH and stronger lubrication; therefore, brittleness of shales soaked by oil-based drilling & completion fluid decreases more evidently. Besides, Longmaxi shale has smaller contact angle, greater imbibition amount, and stronger alkali erosion, the reduction of brittleness is greater than that of Yanchang shale under the same condition. This work can provide some evidence for drilling and hydraulic fracturing in shale formation.