Miscibility of polymer blends around the critical solution temperatures was studied by using a dielectric method for the system composed of low molecular weight cis-polyisoprene (cis-PIp) and polystyrene (PSt) and the system of cis-PIp and vinylpolyisoprene (vinyl-PIp). All these polymers exhibit the dielectric relaxation due to the segmental mode and cis-PIp also exhibits the normal mode relaxation. In the cis-PIp/PSt blend, the normal mode relaxation conformed to the blending law, indicating that the blend is homogeneous over a length scale of the end-to-end distance. On the other hand the cis-PIp/vinyl-PIp system exhibited a complex loss curve mainly due to the segmental mode of vinyl-PIp. The shape of the loss curve varied with temperature, indicating that the blend is weakly segregated and that the degree of inhomogeneity of the length scale of the monomeric unit varies with temperature.