Analysis, design and prediction of the performance of structures made inside or on the rock mass require a proper understanding of the mechanical properties (strength and deformability) of the rock masses. Due to the numerous discontinuities, the statistical nature, their geometric and geomechanical characteristics, it is very difficult to predict the mechanical properties and deformability of the rock masses. The rock strength is the function of mineralogical compounds, cellulose species, grain sizes, how they are placed, the vacant spaces between the grains and the type of rock forming cement. rock strength can be determined in the laboratory or by using in situ tests. Sample size tested, the shape of the sample and the loading rate affect the test results. In this study, the effect of sample size on mechanical properties has been investigated. For this purpose, data on research on concrete and rock samples from different researchers were collected and analyzed. Using artificial neural network method, the value of uniaxial compressive strength was estimated and compared with real values. The results of this comparison showed that the predicted values ??are in good agreement with the actual data. Finally, Finally, concrete samples with different grain size were made, the effect of grain size and angle of discontinuity on sound velocity were investigated. The results showed that by increasing the discontinuity angle, the velocity of the sound increases directly with the uniaxial compressive strength