Strength of rock depends on mineralogical Compounds, grain’s shape and size, Free spaces between the grains and the matrix of the rock. Mechanical properties can be determined through the laboratory and the in situ tests. Specimen dimension and shape and loading rate affect the test results. The specific object of this research is to scrutinize, on the one hand how the dimensions of the grain in the rocks affect the Mechanical properties and fracture behavior of rock, and on the other hand how the dimensions of the specimen affect the results of the test. For this purpose In this regard, at the outset, three concrete panels and nine concrete beam with three entirely distinct grading (12, 20 and 25 mm) have been made, then after processing the specimens, Cores with 56, 68, 72 and 94 mm diameters and with the length to diameter ratio ( ) of 2 have been created. For evaluate of effect of specimen and grain size on the mechanical properties of rock In this research several tests have been applied, including the uniaxial compressive strength test, Four-point bending test to determine the toughness, Brazilian tensile strength test, sound velocity test and the point load strength index test.The results of the compressive strength test indicate that by upsizing the grains, the compressive strength of specimen with diameter 56, 68 and 72 mm also rises. On closer examination, it was clear that such an increase doesn’t occur in specimens with 94 mm diameters and by increasing the grain size, the compressive strength also increases . a comparison between elastic modulus and the velocity of the compressive waves values for specimens with different diameters shows that there is an inverse relationship among the elastic modulus, the velocity of the compressive waves and the diameter. four- point test, it has been observed that by Increasing grain size from 12 to 25 mm toughness and toughness factor Increase. The results of the tensile strength test indicate that changes in the ratio of length to diameter have a considerable impact on the results of this test, that is, an increase in the ratio of length to diameter brings about a decrease in tensile strength. in addition to these, it has been clear that the dimensions of grains have a direct relationship with Brazilian tensile strength. One of the most important result of this research is that by increasing in diameter the compressive strength of a specimen with diameter 56, 68 and 72 mm also increases.