Thermogravimetric analysis (TGA) was applied to investigate the thermal decomposition behavior of poly lactic acid in air with heating rates of 5, 10, 15, 20, 30 K/min and from ambient temperature up to 1250 K. The thermal decomposition features were obtained from resultant TGA curves. The results showed the main thermal decomposition was in the temperature range of 550~660 K and it was found to be the first order reaction from kinetic analysis. Activation energy and frequency factor for thermal decomposition can be estimated by using certain kinetic models.