In the Mohr-Coulomb criterion, the shear strength of sands is typically characterized by the internal friction angle, which depends on many factors such as grain size and distribution, the mineralogical origin of the particles, particle shape, unit weight, geological history, cementation, saturation, and overburden pressure. In this study, the empirical relationships among three particle shape indices, different fractal dimension definitions, and internal friction angles were investigated. Within this context, direct shear tests were conducted on 38 different types of sands from different origins and with various grain sizes. For each type of sand, image analyses were performed to find out the roundness, sphericity, regularity parameters belonging to individual grains. Additionally, several statistics of these parameters for different types of sands were determined. The results revealed that particle shape has a limited effect on the friction angle of sands in comparison to grain size distribution. Furthermore, it was found that decreasing regularity in particle shape caused an increase in the internal friction angle of uniform sands. These findings agree with the empirical relationship between the internal friction angle and particle shape suggested in the literature.