Semi-interpenetrating polymer networks (SIPNs) consisting of polyeaprolactone (PCL) and poly(ethyleneglycol) (PEG) macromer was prepared to improve tensile property in developing biodegradable sutures. When the PEG macromer formed SIPNs with PCL, biodegradability, mechanical strength, and hydrophilicity were improved. The SIPNs fibers formed from the dry spinning process showed an increase of not only tensile strength but also water content with an increase of PEG content. These results represent an increase of the crosslinking density of the PEG network with hydrophobic property. The drawing of SIPNs fibers also further enhanced the tensile strength and the crystallinity of the SIPNs fibers. Unimelting temperature of the SIPNs fiber was observed as an indication of the polymer network without phase separation.