The invention relates to a process for preparing lactic acid polymers of highly crystallinity and molecular weight, the process comprising melt polymerization of a lactide to form prepolymers having active end groups followed by solid state polymerization. The polymerization is carried out in the presence of a catalyst complex comprising a lactide, an organic metal-oxo compound and a lactic acid oligomer. The residual lactide after the melt polymerization is removed by heating the reaction mixture in the temperature range of 98°C to a temperature less than the melting point of the prepolymer. The metal to oligomer ratio in the catalyst complex is in the range of 0.1 to 10, preferably in the range of 0.5 to 5, more preferably in the range of 0.8 to 1.5