Although poly(lactic acid) (PLA) possesses many desirable properties such as miscible, reproducible, nontoxic, and biodegradable properties, extremely slow crystallization rate is a weak point in comparison with other commercial thermoplastics. Addition of nucleating agents can be a good method to increase the overall crystallization rate and multi-walled carbon nanotube (MWCNT) is generally known as a good nucleating agent as well as reinforcement. MWCNT reinforced PLA nanocomposites were prepared by melt blending and the unique nucleation and crystallization behaviors of pure PLA and MWCNT/PLA nanocomposites were investigated. Slow homogeneous nucleation and crystallization behavior of the pure PLA and fast heterogeneous crystallization behavior of MWCNT/PLA nanocomposites were observed. Crystallization behavior of MWCNT/PLA nanocomposites was irrespective of cooling rate and the peculiar behavior was due to fast heterogeneous crystallization caused by the nucleating effect of MWCNT and fast PLA chain mobility.