In this study, two types of nanocellulose were introduced in poly lactic acid (PLA)as nucleation agents, and the crystallization behavior of the composite was studied. Owingto the different aspect ratio, the effect of cellulose nanocrystal (CNC) and cellulose nanofiber(CNF) on the PLA behavior was significantly different. Rheological analysis revealed thatCNC did not affect the chain behavior whereas CNF interfered the chain movement of thePLA. Both CNC and CNF played role as nucleation agents, but the crystallization behaviorsof the composites were different. DSC analysis confirmed that CNC helped to develop αcrystalline of PLA. On the other hand, CNF disturbed the compact crystalline formation andresulted in δ crystalline structure development.