Abstract: The ring opening polymerization of p-dioxanone is started in a liquid phase under stirring at a temperature of 85 to 105° C. after adding 0.002 to 0.005 mol percent of stannous 2-ethylhexanoate forming a white crystal at a temperature of 0° C. as a catalyst and 0.01 to 0.1 mol percent of initiator. Stirring is stopped when a stirring load has increased by 10 to 100 percent over an initial load, while the temperature is lowered to a range of 65 to 85° C., to continue the polymerization in a solid phase to produce poly(p-dioxanone). The polymer is used to make a monofilament having a knot tensile strength of at least 30,000 psi and a strength retention of at least 58 percent after four weeks of immersion in a physiological saline solution at 37° C. The monofilament provides an absorbable surgical suture which is highly resistant to hydrolysis and very useful for stitching a wound or incision which requires a long time for healing.