This study aimed to evaluate the effect of length-to-diameter ratio (L/D) of a rock core specimen on uniaxial compressive strength (UCS) and to investigate the optimum L/D range for different rock types. For this purpose, a considerable amount of rock block was compiled and a series of laboratory tests were carried out to determine some geo-mechanical properties of the rocks and the UCS values of test specimens having different L/Ds. Some assessments were made on the limit L/Ds [(L/D)(lim)] based on theoretically calculated values and those experimentally observed, and then some easily determined properties of the studied rocks were statistically evaluated to predict (L/D)(lim) values. By considering the results of these evaluations, some L/D classes were suggested for selection of the L/D intervals of rock specimens before uniaxial compression tests. In addition, an equation was proposed to correct the UCS values obtained from core specimens shorter than lower limits of the suggested L/D classes.