The shear strength parameters play an important role in many practices for geotechnical engineering. To determine the real values of these parameters requires undisturbed samples and that is difficult for cohesionless soil. So, prediction of shear strength parameter such as angle of internal friction in the case of cohesionless soil with the help of grainsize distribution provides a good opportunity to obtain this parameter without using of more laboratory tests. The grain-size distribution is commonly used for soil classification, design of filters, and ….etc. This study was undertaken in order to quantify the variation of the values of angle of internal friction due to variation of contents of sand soil between fine, medium, coarse sand, or mixtures from some or all of these types. Many types of sand soil have been investigated for this purpose. Shear box tests were conducted using samples of different dry densities. This paper proposed new empirical equations and curves to predict the angle of internal friction for sand soil using the grain-size distribution curve.