The graft polymerization of polylactic acid (PLA) onto methanol-soluble kraft lignin (MeKL) was performed successfully through acylation substitution using chlorine-terminated PLA with a number average of molecular weight (M n) of 5400–24,000 g/mol. As the M n value of the PLA used for the graft polymerization process was increased, the obtained MeKL-g-PLA copolymers decreased from 9,000,000 to 1,100,000 g/mol. The structural characteristics of the MeKL-g-PLA copolymers were analyzed using Fourier transform infrared and 1H nuclear magnetic resonance spectroscopies. It was found that the thermal properties of the MeKL-g-PLA copolymers improved slightly, with their glass-transition temperature increasing to a level higher than that of pure PLA. The tensile strengths and elasticity modulus of the MeKL-g-PLA copolymers were independent of the M n value and were 3.5–3.8 MPa and 0.038–0.045 GPa, respectively.