The fracture toughness of ordinary and refractory concretes in the range of 20-1300 degrees C was investigated, and the stress intensity factor, K-lc, on three-point bent specimens (according to ASTM E-399 recommendation) determined. With.an increase in testing temperature, the stress intensity factor decreases for both concretes. The values of K-lc at 20 degrees C for both concretes are comparable, being equal to 0.64 MNm(-3/2) for ordinary concrete, and 0.72 MNm(-3/2) for refractory concrete, respectively. At 1100 degrees C, K-lc has a value of 0.043 MNm(-3/2) for ordinary concrete, and for the refractory concrete at 1300 degrees C, K-lc = 0.34 MNm(-3/2). The method presented for predicting the behaviour of concrete at high temperature may be used in engineering practice.