A series of novel structured latex particles with semi-interpenetrating polymer network (IPN) of glassy styrene-acrylonitrile copolymer (SAN) cores and rubbery polyurethane (PU) shells were synthesized to improve damping property. The inverted core/shell synthesis method (rubbery part first, then the glassy part) was applied to control morphology of core/shell latex particles. The glass transition temperature and mechanical property of the obtained semi-IPN were characterized by DMA and UTM, respectively. The effect of composition on the mechanical properties of core/shell latex IPN particles was studied. It was found that the core/shell latex particles with high crosslinking density are more miscible than those with low crosslinking density. Additionally, the improved tensile strength and elongation were observed. Therefore, the crosslinking efficiency of PU plays an important role in governing phase miscibility in core/shell latex IPN particles.