Dynamic heterogeneity in miscible blends was investigated by the use of dielectric relaxation spectroscopy for blends composed of poly(vinylethylene) (PVE) and various polyethers, i.e., poly(butylene oxide) (PBO), poly(isopropyl glycidyl ether) (PiPGE), poly(n-butyl glycidyl ether) (PnBGE), and poly(tert-butyl glycidyl ether) (PtBGE). All polyethers are type AB polymers and exhibit both the dielectric normal- and segmental-mode relaxations, but PVE exhibits only the segmental relaxation. The data indicate that the loss curves for the segmental relaxation of PVE/PBO blends are bimodal in the range of PBO content less than 0.25. Since the dielectric relaxation strength of PBO is much higher than that of PVE, unimodal loss curves are seen at high PBO content. The effective glass transition temperatures for the fast and slow modes estimated with the Lodge-McLeish model agree fairly well with the temperatures at which segmental relaxation times become ca. 100 s. The ratio of the relaxation times for the normal and segmental modes log(tau(n)/tau(s)) increases with increasing content of PVE. The correlation length of concentration fluctuation has been deduced to be 1.0 nm from the behavior of broadening of the relaxation spectra for the segmental and normal modes of PBO. In blends of polyethers such as PnBGE/PtBGE, the loss Curves for the segmental modes are unimodal but are asymmetrical when the fraction of one component is 0.25 or 0.75. The asymmetrical loss curve can be attributed to the bimodal relaxation spectrum even though the loss curve is unimodal. The separation between the fast and slow modes in blends of the polyethers has been estimated by utilizing log(tau(n)/tau(s)) as a tool to estimate the location of the segmental loss peaks of the components.