The characteristics of micelle formed in blends of styrene-isoprene diblock copolymer (SI) and poly(vinyl methyl ether) (PVME) were investigated by using small angle X-ray scattering (SAXS), transmission electron microscopy (TEM), and light scattering techniques. The results of SAXS and TEM experiments indicate that the blends form spherical micelles with isoprene-rich cores and styrene-corona mixed with PVME in a matrix of PVME. As temperature increased, the blends underwent macrophase separation into a PVME-rich phase and a SI-rich phase which was also microphase-separated to ordered microdomains. Upon further heating, the ordered structure of the SI-rich region transforms to a disordered phase in a manner resembling the order-disorder transition in pure block copolymer melts.