We examined temperature(T)- and molecular weight(M)-dependent changes of the phase separated structure in immiscible polymer blends. Blend samples used were polyisoprene(PIP-H: Mw = 19.9x10(3); PIP-L: Mw = 14.4x10(3)) and polydimethylsiloxane(PDMS: Mw = 1.20x10(5)). The blend ratio was PIP/PDMS = 8/2(wt/wt). These blend samples included discrete phases(droplets) of PDMS and showed the relaxation much slower than the component relaxation. This relaxation reflected the shape recovery of the droplets, and the PIP/PDMS interfacial tension can be estimated with Palierne model. The interfacial tension decreased with increasing T, and this decrease was bigger for PIP-L than for PIP-H. These results agreed with the T and M dependencies of the UCST-type polymer-polymer compatibility.