Soil has been used as a construction material for long time, and therefore is a subject of engineering research. The soil is formed by the decomposition of the base rock, and it is divided into : coarse or fine grained soils. Behavior of coarse-grained soil is largely different from the behavior of fine-grained soil. In reality it is a very rare to find soil made of one type of fracture (sand and clay), it rather consists of mixtures of fractions. Research has shown that there is a limit in percentage of mass in mixture of sand and clay where clay particles dominate with it's behavior over sand. For the purpuse of this work a total of 18 direct shear tests have been conducted. The goal of this work is find out how fine grained particles, clay, affect on strenght of sand. A total of 9 series were conduced on pure sand, and sand mixed with different particles of clay, and with different moisture levels. Each of the series consisted of three samples that were loaded with three different normal stresses. All results are shown graphically by diagrams of horizontal displacement and force, horizontal displacement and vertical displacement, horizontal displacement, and shear strengt, and in the end relation of normal stress and shear stress. All tests were conduced in Geotechnical laboratory of Civil engineering collage in University of Rijeka.