Abstract: Hydraulic fracturing is the key technology in the exploitation of unconventional resources,such as shale gas. Depth of the shale reservoir in the southwestern area of Sichuan Basin has breached 4000 m and some blocks even are approaching 5000 m. There are great differences in the exploitation of such reservoirs due to the challenge in situ conditions in the deep subsurface. Brittleness of the reservoir rock under the in situ conditions plays very important role in the hydraulic fracturing of the reservoir. Shale samples were collected from the borehole and in situ conditions were recorded on the drilling site. The standard rock specimens were prepared,having a height to diameter ratio of 2.0 and a diameter of 50 mm. Triaxial compression experiments under in situ conditions were carried out in the laboratory to obtain the complete stress-strain curves. Brittleness index based on the complete stress-strain curves under in situ conditions were used here for the evaluation of brittleness of the reservoir rocks. Brittleness of four sub-layers of the Longmaxi shale and underlying Wufeng shale were estimated using the brittleness index under in-situ conditions. Results of the brittleness values of the reservoir rocks were studied and the comparison with the fracturing results of the real reservoirs is carried out. The results of brittleness evaluation under in situ conditions agree well with the fracturing operation on the drilling site.