Hexabutyl guanidinium acetate (HBG•OAc) was synthesized and successfully used as a catalyst for the ring-opening polymerization (ROP) of lactides. The experimental results indicated that the guanidinium salt HBG•OAc showed satisfactory catalytic behavior. Polymerization in bulk (120℃, 18h) produced polylactides with moderate molecular weights (number-average molecular weight=2.0×104) and very narrow molecular weight distributions (polydispersity index=1.07-1.12). A kinetic study of polymerization in bulk with HBG•OAc as an initiator revealed that the polymerization possessed typical characteristics of living polymerization. A ROP mechanism by HBG•OAc was proposed on the basis of the additive effect of the polymerization and the 1H NMR characterization of the microstructure of the product polymers.