A fully biodegradable ternary poly(L-lactide) (PLLA)/poly(diethylene glycol adipate) (PDEGA)/cellulose nanocrystal (CNC) composite was prepared with the objective of enhancing the crystallization of PLLA, which was significantly promoted due to the synergistic effect by the combination of PDEGA as a plasticizer and CNC as a nucleant. From the lowest cold crystallization peak temperature, the highest melt crystallization peak temperature, and the shortest crystallization half-time of ternary PLLA/PDEGA/CNC composite, PLLA crystallized faster in the presence of both PDEGA and CNC simultaneously than in the presence of only PDEGA or CNC, although only plasticized PLLA or nucleated PLLA crystallized faster than neat PLLA.