Dielectric study was performed on the polyisoprene (PI)/poly(vinyl ethylene) (PVE) miscible blends. We observed the end-to-end vector fluctuation dynamics (normal mode) of PI as well as the two alpha-relaxation processes corresponding to the segmental motions of PVE and Pl. Among these three relaxation modes, the normal mode relaxation behavior was examined in detail. Since this relaxation reflects the dynamics of the length corresponding to the end-to-end distance of PI chain, comparison of the relaxation times for the normal mode and alpha-processes gives the information about the length scale of the two alpha-relaxation modes. We estimated the relevant length scales for the segmental motions in the blend and compared them with a single cooperative volume model and a dual length scale model. As a result, the latter model was found to describe better the features of the segmental dynamics in PI/PVE blends.