Poly(lactic acid) is the biodegradable aliphatic polyester produced in small quantity and in large quantities industrially and used very variously. Because of its good mechanical property and reconcilability with environment, it is one of the polymers most appropriate to replace non-biodegradable synthetic polymers based on crude oil. PLA can be made from lactic acid with different polymerizations such as polycondensation, ring-opening polymerization, and direct polymerizations by azeotropic dehydration or the enzymatic polymerization. The direct polymerization and the ring-opening polymerization are the most used techniques. This paper reviewed methods of making PLA such as oligomer polycondensation or ring-opening polymerization and mainly mechanism of ring-opening polymerization with 4 areas: metal catalyst, organic catalyst, cationic catalyst and stereo-controlled polymerization.