The glass transition behavior of ternary blends of polypropylene (PP), polystyrene (PS) and styrene-ethylene-propyl ene-styrene block copolymer (S EP S) was investigated. The blends were prepared by an internal mixer, and their dynamic mechanical properties and morphology were measured. The blends showed phase inversion at around 75wt% PS composition. The glass transition temperature (Tg) of the PP phase shifted to lower temperature as the PS contents were increased in PP/PS binary blends, probably due to the mismatch of thermal expansion coefficients between two components. As the SEPS copolymer contents were increased, the Tg＇s of the PP phase in the blends increased. In particular, the large increase in Tg of the PP phase was observed in the PP/PS (25/75) blends where the phase inversion takes place.