This study examined the reinforcing effects of multiwalled carbon nanotubes (MWCNTs) with different morphologies in a poly(L-lactic acid) (PLLA) matrix. The surfaces of the MWCNTs were modified using linear alkyl chains to disperse the MWCNTs homogeneously in the polymer matrix. The morphology of the MWCNTs was determined from numerical values for the $D_b$, $l_{sp}$ and $I_G/I_D$. These numerical values were related to the morphology of the MWCNTs and had an effect on the PLLA/MWCNT composites. The rod-like MWCNTs with a relatively high $D_b$, $l_{sp}$ and $I_G/I_D$ exhibited superior electrical and thermal properties owing to their morphological characteristics, such as their more crystallized $sp^2$ carbon structure, high contour length and static bending persistence. Therefore, rod-like MWCNTs are more advantageous than those with a bent morphology with respect to MWCNT reinforced polymer composite applications.