Effects of carbon nanotubes (CNT) on the dynamic mechanical property, thermal property, and crystal structure of poly(L-lactic acid) (PLLA) were investigated. Dynamic mechanical analysis (DMA) found that CNT via grafting modification with PLLA (CNT-g-PLLA) could result in effective reinforcing effects. Tan delta of DMA found that CNT-g-PLLA was compatible with the PLLA matrix, giving a single T-g of the composite with a higher CNT-g-PLLA loading giving a higher T-g of the composite. Wide angle X-ray diffraction (WAXD) data demonstrated that CNT could assist the disorder-to-order (alpha'-to-alpha) transition in PLLA crystals but did not lead to a more compact chain packing of the crystal lattice in PLLA composites than in pure PLLA. The equilibrium melting temperature (T-m(0)) obtained from Hoffman-Weeks plots were found to increase with increasing CNT-g-PLLA content. Small angle X-ray scattering data revealed that thicknesses of crystal layer and amorphous layer of PLLA both decreased with increasing CNT contents. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 145-152, 2010