Fracture toughness is an indicator of resistance to crack propagation and one of the most important parameters of fracture mechanics of rocks and other solids. Various tests have been presented to determine the Mode I fracture toughness of rocks, but their results differ from each other in practice. In this paper, two tests, a cylindrical specimen with a straight groove and a disk specimen with a straight groove, were selected from the various methods for determining the fracture toughness of rocks in Mode I to determine the difference in fracture toughness values. The results indicate a difference of 13.5% between the fracture toughness values ​​of these two tests, which was not a very large difference. In order to investigate the causes of the difference in fracture toughness values, numerical modeling of these two tests was performed using the three-dimensional finite element method. The results of numerical modeling indicated that the volume of the plastic zone of the crack tip was the same in these two tests, as a result, the accuracy of the linear elastic theory for both tests is the same. Another factor that was discussed to investigate the reason for the difference in fracture toughness values ​​in these two tests was the ratio of groove thickness to sample length. Given that this ratio is lower in cylindrical samples with a straight groove and, as a result, the assumption of the groove as a crack in these samples is more correct, the cylindrical sample test with a straight groove provides more reliable values ​​of fracture toughness.