Based on the double K criterion, the residual Mode I fracture behaviors of concrete subjected to elevated temperatures were investigated. The test specimens were molded according to “Norm for fracture test of hydraulic concrete”（DL/T 5332—2005） which were exposed to temperature varying from 65℃ to 600℃. The mass losses of all the specimens were recorded. The wedge splitting method was employed to obtain the p(load) CMOD（crack mouth opening displacement） curves of post fire specimens from which the initial fracture toughness Kini, the critical fracture toughness Kic and the fracture energy GF were calculated. It was demonstrated that the Kini decreases monotonously with increasing temperature; however the Kic and GF sustain a hole increase decrease tendency with temperature. Furthermore, the characteristic length as a brittleness parameter was also calculated and there exists a same tendency with Kic and GF, which meant that these three parameters are similar as the index of brittleness for post fire concrete.