The soil shear strength is the maximum shear stress that a soil can withstand during its breakdown. One of the important parameters of shear strength is a peak friction angle which is determined in a laboratory. In addition to laboratory tests, the angle of internal friction can be assumed by using existing correlations, such as a plasticity index correlation. The topic of the bachelor thesis is the analysis of the relationship between a peak friction angle and a plasticity index. The results of measuring samples at the Faculty of Geotechnical Engineering in Varaždin were used for the analysis. All measurements were performed on a direct shear device with constant deformation. A lot of clay soil samples of different plasticity indexes were examined. The plasticity index was calculated as the difference between the liquid limit and the plasticity limit obtained by laboratory tests. Sampling and testing of materials has been carried out during the last 20 years, and the material has been taken mostly from the locations in the continental Republic of Croatia. In addition to the analysis of the samples, the basic concepts important to the analysis were explained and a brief review of the existing research was made. The performed tests and measurement results confirmed the assumed weak negative correlation between the parameters of a peak friction angle and a plasticity index which were obtained by many other researchers, such as Ortolan and Mihalinec. A weak correlation of this type is insufficient to conduct further analyzes or draw certain conclusions about a soil, but it can help as an additional tool in preliminary analyzes and assumptions about it.