The mechanism of propagation and arrest of a brittle crack, which was governed by the plastic surface energy, the temperature, the crack length and the applied stress, was investigated from a viewpoint of crack speed. The ESSO test with temperature gradient was conducted and analyzed theoretically. A crack propagating in a field of gradient temperature reduces its speed gradually with the increasing crack length, and then is arrested abruptly after the speed downs to lower than the critical crack speed of 250_??_400m/sec. The effect of shear lip on the plastic surface energy was found by using a specimen with hardened surface. The stress-arresting temperature curve for a given temperature distribution was approximately estimated from the crack speed data under a stress level.