A practical method for the racemization of L-arginine and an enzymatic method for optical resolution of DL-arginine by L-arginine amidinase have been developed. We obtained D-arginine and L-ornithine hydrochloride after isolation and purification. Many factors affected the racemization and enzymatic reaction. The whole process of racemization by this method can be completed within 2.5 h. The ratio of the racemization was as high as 74.5%. The result shows that the optimum conditions of enzymatic reaction were: 35 ℃, pH=10, c( DL-arginine) = 40 g/L and c(L-arginine amidinase) = 0.2 g/L. The conversion rate of L-arginine was higher than 98%. The products were separated and purified on ion-exchange resin. The optical purity of products were 99%.