Based on the fact that the fractured zone in rock mass contains large numbers of tensile cracks and these cracks can grow at different velocities under different loading patterns, the effect of the grain size and mineralogical composition on the rupture speed and propagation path of these cracks was investigated. Three rock types of granite and hornfels that differ in grain size and mineralogical composition were chosen to achieve the goal of this study. Physical and mechanical properties of these three rock types were determined. A simple and inexpensive electrical method was developed to measure the rupture speed. After the fracture, two sections were prepared from the SCB specimens in order to analyze the fracture path. The first section was parallel to the notch whereas the second section was perpendicular to it. The results indicated that the rupture speed, fracture toughness, tortuosity factor and the ratio of intergranular to intragranular fractures are inversely proportional to the grain size. The width of the fracture path increases with increasing the grain size. This study proved that the fracture path is firstly affected by the stress path between the notch tip and the applied load, and secondly by the grain size and mineralogical composition. If the boundary between two grains agrees with the stress path, the fracture will grow along this boundary, otherwise, the fracture will split one of the two grains to remain in the stress path. By increasing the grain size, the number of intragranular fractures increases and due to the difference in impedance between grains, the fracture will deviate from its original path when it moves from one grain to another. Therefore, the number of deviations increases with increasing the grain size and as a results, the tortuosity factor and the rupture speed decrease.