Unsaturated polyester/polyurethane interpenetrating polymer networks (UP/PU IPNs) were prepared by varying the UP/PU weight ratios. Atomic force microscopy was applied to investigate the surface topographic images of IPNs with various UP/PU ratios: 100/0, 80/20, 70/30, and 50/50. The surface of pure UP was very flat. With increasing PU content, the surface appeared heterogeneous with a roughness of tens of nanometers, and in addition, PU moieties were preferentially dispersed on the surface of the UP matrix in the form of dumps or circular plates with sizes ranging from nanometer to micrometer order, as indicated by the occurrence of phase separation.