The plasticization effect of poly(ethylene adipate) (PEA) with relatively low and high molecular weights on the crystallization behavior and mechanical properties of biodegradable poly(L-lactide) (PLLA) was investigated. Despite the molecular weight of PEA, both the single and decreased glass transition temperature and the depression of equilibrium melting point of PLLA confirmed the miscibility between the two components. The plasticization effect of PEA with low molecular weight was more efficient than that of PEA with high molecular weight. Consequently, the crystallization rate and spherulitic growth rate of PLLA plasticized by PEA with low molecular weight were faster than those of PLLA plasticized by PEA with high molecular weight. The mechanical properties of PLLA plasticized by PEA with high molecular weight were better than those of PLLA plasticized by PEA with low molecular weight. The crystallization behavior and mechanical properties of PLLA could be adjusted by controlling the molecular weight of PEA.