Abstract: According to the crack expansion problem of the rock in the drilling, blasting, tunnel excavation and other engineering, the modified ring specimens were applied to measure the Type I fracture toughness of the rock. The Griffith Strength Criterion was firstly applied to measure the initial cracking location of the specimens in the loading diameter direction under the radial loading function, then the finite element method was applied to calibrate the max dimensionless stress strength factor of the modified ring specimens. Finally the analysis on the load- displacement curves and crack expansion process and the selection of the coal min load were conducted. The study results showed that a maximum of the equivalent Griffith stress could be obtained at the crossing point between the loading diameter and the borehole edge and the central cracking of the specimen could be ensured. The displacement would be reduced with the flattened angle increased and would be increased with the borehole diameter reduced. The test results and the theoretical analysis could be basically fitted. The fracture toughness value approved of the fracture toughness value obtained from the modified ring specimen was reliably. Thus based on the dimension law provided by BAZANT, a preliminary discussion was conducted on the modified ring specimens applied to the dimension effect of the fracture toughness test.