The amount of organic matter in clay soil affects its physical and mechanical properties. Mixing clay with different levels of organic matter showed that specific gravity and void ratio changed linearly, while water content, Atterberg limit, shear strength, cohesion, and friction angle changed nonlinearly. At 7.5% organic matter, there is a turning point for moisture content and Atterberg limit. Shear strength properties change at 7.5% and 37.5% organic matter levels. Below 7.5%, organic matter is bound to clay; between 7.5% and 37.5%, it is mineral-free; above 37.5%, it is free organic matter. The interaction between minerals and organic matter affects the shear characteristics of clay differently at different organic matter levels.