<p>The heterogeneity of the grains is one of the most important components in the petrographic studies of the rocks. In this research work, the effect of grain heterogeneity on the relationships between the grain shape characteristics and the uniaxial compressive strength (UCS) of eight specimens involving four granites, three travertines, and one marble has been investigated. For this purpose, microphotographs were taken using an optical microscope from thin sections and the grain shape characteristics including aspect ratio, shape factor, roundness, solidity, and compactness were calculated using the image processing technique. Then, different degrees of grain heterogeneity of rocks (Iteration number, I = 10, 200, 500, and 1000) were simulated using UDEC software and the grain shape characteristics were also calculated. In the next step, the relationships between the average values of the grain shape characteristics and the UCS of the rocks were determined using regression analysis. The results showed that with the increase in the solidity and compactness, the UCS of the rocks increased. Moreover, with increasing aspect ratio, roundness, and shape factor, the UCS of the rocks has decreased. Also, based on the regression analyses, it was found that the type of relationships between the grain shape characteristics with the UCS of the rocks and the values of R² based on four degrees of grain heterogeneity are different. In general, the role of grain heterogeneity is inevitable in investigating the relationships between petrographic characteristics and physicomechanical properties of rocks.</p>