The purpose of this research was to study the thermal degradation kinetics of nanocomposites of poly(l-lactide) (PLLA) with carbon nanotubes (CNT) in order to provide further insight into their thermal stability. Nanocomposites were prepared by solvent casting with 1, 3, and 5% by weight of pristine CNT (P-CNT) or functionalized CNT (F-CNT), and were characterized using infrared spectroscopy, transmission electron microscopy, differential scanning calorimetry, thermogravimetric analysis, and dynamic-mechanical-thermal analysis. The kinetic parameters of thermal decomposition were determined employing Coats-Redfern method to calculate the reaction order and E-2 function model to calculate the activation energy (Ea). We found no major changes in PLLA glass transition temperatures due to CNT presence, but melt-crystallization temperature increased slightly in some composites. In general, composites consisting of 3% or 5% of F-CNT had superior thermal stability than did pure polymer or P-CNT composites. This improved thermal stability was revealed by slightly higher degradation and onset temperatures, and Ea obtained from kinetic analysis. In addition, 3% or 5% of F-CNT in PLLA composites slightly enhanced the storage modulus above the glass transition. Therefore, functionalization promoted, in some extent, better morphology and dispersion of CNT into the matrix, which was responsible for improved thermal stability and thermomechanical performance of composites at higher temperatures relative to pure polymer. POLYM. ENG. SCI., 55:710-718, 2015. (c) 2014 Society of Plastics Engineers