Fracture toughness of Antrim shale cores from Dow Chemical's Sanilac County test site in Michigan were measured by the burst test method developed by Clifton et al. (1976). These tests were conducted to establish a preliminary data base to be used for the designing of a bed preparation method and prediction of rock fracture behavior under various loading conditions such as explosives and hydraulic fracturing for in-situ processing of oil shale. The test method was chosen because the thick-walled cylinder provides a loading and specimen configuration similar to in-situ hydraulic fracturing operations and the specimens can be conveniently prepared from diamond drill cores for laboratory tests. Further, the nature of variation of crack tip stress intensity in this specimen is such that K/sub IC/ does not depend on initial crack length, and crack propagation need not be monitored. The test results show that the fracture toughness of typical Antrim shale core range from 930 to 1080 psi ..sqrt..in. while the limestone specimens, a basement rock, range from 1240 to 1430 psi ..sqrt..in. These values are close to that of lean Western oil shale from Anvil point, Colorado (Schmidt, 1977), i.e., 980 psi ..sqrt..in.