Poly (p-dioxanone)(PPDO)/montmorillonite nanocomposites were prepared through the in situ ring-opening polymerization of p-dioxanone (PDO) and three types of montmorillonites (natural sodium montmorillonite, montmorillonite modified by octadecyltrimethyl ammonium chloride, and montmorillonite modified by hydroxyethylhexadecyldimethyl ammonium bromine) in the presence of triethylaluminum. Montmorillonite could accelerate the polymerization of PDO, and the viscosity-average molecular weight of PPDO could reach 44,900g/mol in 0.5h. A nucleating effect of montmorillonite was observed, and the crystallization temperature of PPDO was increased by 18 8C. All three montmorillonites could improve the thermal stability of PPDO and increase the glass-transition and melting temperatures of PPDO.