Nanocomposites of poly(L-lactic acid) (PIA) with oxidized multi-walled carbon nanotubes (MWCNTs-COOH) were prepared by solved evaporation method containing 2 5 wt% MWCNTs-COOH From the thermogravimetric curves It can be seen that PLA/MWCNTs-COOH nanocomposite present a relatively better thermostability than PLA. The activation energy was calculated with the isoconvertional Ozawa-Flynn-Wall and Friedman's methods For all the mass conversions, PIA has lower values than the nanocomposite From the variation of the activation energy (E) with increasing degree of mass conversion especially for the PLA it was concluded that the decomposition of all the samples is taking place with a complex reaction mechanism with the participation of at least two different mechanisms The best fitting of experimental data with theoretical models for PIA and PLA-2 5 wt% MWCNTs-COOH give nth-order for the first mechanism and nth-order with autocatalysis (Fn-Cn) for the second mechanism with different activation energies (C) 2010 Elsevier B V All rights reserved