Melt/solid polycondensation (MP/SSP) of poly(c-lactic acid) (PLLA) was conducted by using several aromatic sulfonic acids as the catalysts. It was clarified that the increase in molecular weight and the thermal stability of the obtained polymer are correlated with the 5% weight loss temperatures (T-d,T-5%) of the sulfonic acids determined by thermal gravimetric analysis. When 2,5-dimethyl-, 2,4-dimethyl- and 4-methyl-benzenesulfonic acids showing T-d(,5%) around 150 degrees C were used as the catalysts, PLLA polymers with high molecular weight and excellent thermal stability could be obtained. On the other hand, 2,4,6-trimethylbenzenesulfonic acid showing T-d(,5%) around 100 degrees C and 4-dodecylbenzenesulfonic acid with T-d(,5%) above 200 degrees C gave PLLA polymers with remarkably low thermal stability. The residual catalyst involved in the product polymer or SO3 generated during the polymerization decreased the thermal stability of the polymer. The oligomeric fractions of the PLLA products were extracted with acetone and analyzed by H-1 NMR and mass spectra. It was verified that the catalyst had efficiently been removed by thermal annealing while it remained active during the SSP to produce a thermally stable PLLA with high molecular weight.