A series of interpenetrating polymer networks (IPNs) composed of hydrophilic polyurethane (PU) and hydrophobic polystyrene (PS) were prepared by the simultaneous polymerization method. The PU network was synthesized via the isocyanate-terminated PU prepolymer based on polyethylene glycol (PEG), a highly hydrophilic oligomer, and hexamethylene diisocyanate (HDI). The bulk and surface characteristics of these materials were analyzed by differential scanning calorimetry (DSC), tensile testing, scanning electron microscopy (SEM), attenuated total reflectance-Fourier transform infrared (ATR-FTIR), electron spectroscopy for chemical analysis (ESCA), and contact angle measurement. The PU/PS IPNs prepared in this study exhibited phase separated structures, which had dispersed PS domains in the continuous PU matrix, in both the bulk and surface showing two transition temperatures. The IPN containing 50 wt% of PS showed good mechanical properties. The enrichment of PU phase in the surface was revealed by SEM, ATR-FTIR, ESCA, and contact angle measurement.