Stress intensity factors of specimen models with various external diameters, inner hole diameters, and extents of flattened loading boundaries for the modified ring testing method for mode I fracture are computed by finite element modeling. Effects of boundary conditions and geometric factors on stress intensity factor computation are analyzed. Parametric expressions are proposed to estimate critical crack length position and corresponding maximum stress intensity factor. An expression in terms of inner hole and specimen diameter, and loading angle is developed for estimation of maximum stress intensity factor. Mode I fracture toughness tests with modified ring method are conducted on two different rock types; andesite and marble. Results of the modified ring tests with different external specimen diameters are compared to the results obtained by the cracked chevron notched Brazilian disk method. Geometric parameters of modified ring specimens yielding fracture toughness results close to the results of the suggested method are identified. Fracture toughness of large diameter specimens with small inner hole sizes is found to be higher than the results of the suggested method. A similar trend is observed with decreasing width of loaded boundaries. Effects of specimen and inner hole size, and loaded and free boundaries on fracture process zone and fracture toughness are discussed. (C) 2012 Elsevier Ltd. All rights reserved.