The thermo-mechanical properties of poly (l-lactide) (PLLA) biodegradable polymer reinforced with PLLA grafted from multiwalled carbon nanotubes (MWCNT-g-PLLA)s are characterized. The crystallinity of PLLA polymer matrix affected by MWCNT-g-PLLAs is illuminated. For this purpose, the PLLA chains are covalently grafted from the sidewall of aminated MWCNTs. Then, the MWCNT-g-PLLAs/PLLA composite films are prepared by solution casting using chloroform as solvent. It is found that the MWCNT-g-PLLAs well dispersed in PLLA matrix. The mechanical properties of PLLA enhanced gradually with the increasing concentrations of MWCNT-g-PLLAs up to 2 wt%. The MWCNT-g-PLLAs increase the glass transition temperature (T (g)) and melting point of PLLA as revealed by the curves from differential scanning calorimeter (DSC). In addition, the dynamic mechanical analysis (DMA) results show that the T (g) and Young modulus of PLLA increase with the increment in the concentrations of MWCNT-g-PLLAs. Due to the homogenous dispersion of MWCNT-g-PLLAs and the van der Walls force between grafted PLLA chains on the sidewall of MWCNTs and the PLLA matrix chains, the chain stiffness in amorphous phase of PLLA increases. In addition, the MWCNT-g-PLLAs as heterogeneous nucleation agents increase the crystallinity of PLLA.