The paper presents a brief summary of an experimental testing obtained by a direct shear box. In this study, two series of direct shear tests and interface shear tests are conducted to investigate the effects of grain size and gradation on the ratio of skin friction angle between soil and stainless steel to the internal friction angle of calcareous sand. The differences in stress – strain relationships for the samples obtained by both types of tests are identified. Although, the shear stress reaches a peak state at large shear strain, the interface shear stress reaches a peak value in an early state of shearing and then fluctuate harmonically until the end of shearing. The fluctuation is found greater for the samples loaded at higher applied normal stress. The results indicate that the ratio decreases with the increase in D50 and increases with an increasing uniformity coefficient (Cu). This tendency can clearly be seen in the samples tested at higher normal loads. For both effects, the ratio is found to vary in a rather limited range of 0.26-0.50 corresponding to the range of interface friction angles of 11o-17o. These information may be useful and necessary at least for predicting the shaft bearing capacity of piles embedded in calcareous sand and for other practical applications in foundation design