Poly (L-lactide) (PLLA) with molecular weight ranging from 7600 to 700 000 has been purified from traces of residual metals and has then undergone thermal behaviour examination. The PLLA decomposition temperature obtained by thermogravimetric analysis was used as a measure of its thermal stability. It has been demonstrated that residual low molecular weight compounds such as moisture, lactic acid, lactide, etc. associated with PLLA can be removed without affecting the polymer thermal stability, while residual metals were revealed to be the principal parameter which causes drastic thermal degradative effect on the polymer. The decomposition temperature of pure PLLA at infinite molecular weight was 353 degrees C. The thermal stability of PLLA has been studied as a function of the amount of added tin, zinc, aluminium and iron. Experimental results enable the comparison of their influence on PLLA thermal stability which follows a decreasing order : Fe > Al > Zn > Sn. The more selective the catalyst for PLLA polymerization, the less efficient is its depolymerization effect at high temperature.