In this study, the effect of particle size and nonuniformity of sediment mixture on local scour of cohesive sediments with different clay contents is investigated experimentally. Nonuniform graded and three uniform sediments with seven clay contents were used in the experiments. The results indicate that the particle size affects the characteristics of local scour hole significantly, depending also on the clay percentage. However, for high values of clay in the sediment mixture (Cc > 0.2), the effect of sediment nonuniformity is more important. Also in the experiments with non-uniform sediments and clay content in the range of 0.25–0.3, the maximum scour depth, dsm, was reduced because of increase of grain size geometrical standard deviation and formation of armored layer at the upper surface of the scour hole. Finally, the effect of tailwater depth on dsm in cohesive sediments is studied and a simple equation is proposed to determine the maximum scour depth for different sediments without considering the effect of grain size where high precision is not expected.