Abstract: The effect of anionics-oily additive on the seed precipitation from sodium aluminate solution was studied. How the particles strength and size and the rate of precipitation of Al(OH)3 are affected by adding fatty acid and heneicosane with different proportion and quantities. The results showed that when the mass fraction of fatty acid of the oily additive is in the range of 15% to 49% with the additive concentration in sodium aluminate solution in the range of 50×10^-6g·L^-1 to 150×10^-6g·L^-1, the particles size of A1(OH)3 increases with the quantity of fine particles reduce so as to improve the particle strength, but the rate of precipitation is reduced slightly. However when the mass fraction of fatty acid in the additive is 32% and its concentration in sodium aluminate solution is 150×10^-6g·L^-1, the comprehensive property of the additive proposed is better than that of similar foreign products.