In the field of fracture mechanics, it is of great importance to obtain the stress intensity factor of a crack when the cracked material is under a load. In this paper, we report the results of a case of a block specimen with a crack under transient heating loads from both sides of the block. We experimentally inventigated the stress intensity factor for a block that is heated abruptly under a certain constant temperature. For the purpose of comparison, both caustic and photoelastic methods are employed and the effects on the stress intensity factor of block dimensions are considered and discussed.