Brittleness index (BI) is a key parameter used for reservoir fracability evaluation and "sweet spot" search in shale gas exploration. The existing assessment methods cannot guide the fracturing design and field treatment due to their insufficient theoretical foundation and neglect on the influences of engineering factors. This study aims to solve this problem by using the thermodynamics criterion in nonlinear fracture mechanics. First, the quantitative relationship between energy loss and rock brittleness was investigated during fracture propagation. The nonlinear plastic deformation at the crack tip was then simplified into an ideal line elastic condition by using the equivalence principle. In addition, the numerical equation for surface pressure was fitted by the monadic regressive method. A case study was conducted to compare the obtained BI using common methods and the proposed one. Results demonstrate that energy dissipation theory is recommended to describe the essence of nonlinear rock deformation. Moreover, a higher BI indicates more occurrences of microseismic events. The monitoring data fitted with the conclusion well, which verify the applicability of the proposed evaluation method. Furthermore, the average BIs of the two wells used in this study are 48% and 62%, which shows obvious positive correlation with production. The proposed model can be regarded as a good reference for BI evaluation in shale reservoirs. Keywords: Brittleness index, Shale gas, Hydraulic fracturing, fracability