Novel amphiphilic block polymers composed Of poly(L-lactic acid) (PLLA) and poly(sarcosine) (PS) were synthesized by successive polymerizations of lactide and sarcosine N-carboxy anhydride (NCA). The degree of polymerization of each block was controlled by the monomer/initiator ratio in feed. The block polymers were successfully dispersed in buffer with taking various morphologies of micelle, lamellar, and vesicle depending on the hydrophobic/hydrophilic balance of the copolymers. The size of the molecular assemblies was in the range from 20 to 200 nm, which may be available for drug delivery system (DDS) as nanocarrier.