Mechanistic aspects of lipase-catalyzed ring-opening polymerization (ROP)of lactones to give polyesters are discussed from accumulated experimental data and new insight. Comparison of the ROP reactivity by lipase catalyst with the anionic ROP reactivity by a metal-catalyst clearly demonstrates the characteristics of lipase catalysis; the larger ring-sized monomers with lower ring strain showed higher polymerizability than medium ring-sized ones, in contrast to the anionic ROP showing the reverse direction where the ring strain of monomer is operative. The enzyme-catalysis involves an acyl-enzyme intermediate formation as a key-step. From the copolymerization results a new mechanism is proposed, that involves the formation of the acyl-enzyme intermediate (acylation step)and/or the nucleophilic attack of the propagationg alcohol end to the carbonyl carbon of the intermediate to open the monomer ring (deacylation step)as the rate-determining step. The structure of the propagating alcohol end (primary or secondary)affects much on which step is more operative.