A synthesis of Dimer acid was studied from a tall oil fatty acid. Catalytic activity measured as reactant conversion in a autoclave reactor increase in accordance with the acidity. The optimization of process conditions were tested by an experimental design method. Optimization synthetic conditions of dimer acid and were reaction of tall oil fatty acid during 2 hour at $250^{circ}C$, used of 7.3 wt% active clay and $1.2{sim}1.4wt%$ water, and found reation pressure $8{sim}9Kg/cm^2$. The maximum conversion rate was researched $74{sim}76%$.