Abstract: The direct shear test measures shear strength parameters of soils and other particulate materials. In this study the effects of box size on the strength parameters of soil is studied in Isfahan city area. The soil samples are classified as SC according to the unified soil classification system. The samples were well graded with dry density ranges in the between 1.67 and 1.82 g cm-3. The tests on the shear strength properties of SC soils were carried out by using large, medium and small scale direct shear equipment with shear box dimensions of 60, 100 and 300 mm. The undistributed samples, having almost the same properties, were tested in three square shear boxes of varying sizes. Forty five sets (each set with three samples) of direct shear tests at a constant rate of 1 mm min-1 were performed to study the influence size of the shear boxes and soil density on the strength parameters. The results show the effect of scale on the test. The large and medium scale direct shear produces a higher cohesion and lower friction angle compared with the results of the small-scale direct shear test. The tests indicate that the friction angle and cohesion increase when soil density in each of the three boxes increases. Therefore, these observations suggest that strength parameters are controlled by the scaling effect and physical properties of the soil. The present study also shows the relationship between the shear strength parameters of small and large scale direct shear tests.