In this work, influence of carbon nanotubes modification on isothermal crystallization kinetic of polylactide was investigated. Multiwall carbon nanotubes (MWCNT) modified with -COOH group and -COOC12H25 group were used. PLA/ MWCNT nanocomposites (0-5 % wt) were prepared by melt blending procedure in Brabender plasticoder and by molding in press. Crystallization kinetic was investigated using differential scanning calorimetry. Results were analyzed according to the Avrami method and the parameters n and k were determined. Parameter k is proportional to the density of nucleation and to rate of crystal growth while parameter n depends on nucleation and crystal growth mechanisms. Using the following parameters t1/2, tmax and K that are indicators of crystallization rate are calculated and analyzed. Vales of n parameter, between 2 and 3.3 indicate that part of the crystals has three-dimensional spherulite structure and part of them two-dimensional.Values of parameter K for PLA matrix indicate that rate of crystallization increases up to 100 °C, and after that decreases. By filler addition temperature of maximal crystallization rate is shifted toward lower temperature. MWCNT-COOH and MWCNT-COOC12H25 accelerate isothermal crystallization of PLA matrix, meaning that they have nucleation effect. Nucleation effect of these fillers depends more significantly on the temperature of isothermal crystallization and on amount of added filler, and slightly on type of functional groups grafted on MWCNT's surface. The highest crystallization rate has PLA nanocomposite with 5 % wt of MWCNT-COOH at temperature 95 °C.