It has been known that blends of poly(epsilon-caprolactone) (PCL) and poly(styrene-ran-acrylonitrile) (SAN) show a miscibility window and, at the edges, lower critical solution temperature (LCST) behavior occurs. The presence of a virtual upper critical solution temperature (UCST) of PCL/SAN blends was found at lower temperatures as follows. Time-resolved light scattering under V(v) and H(v) optical alignments was used in order to distinguish two competitive processes-crystallization and liquid-liquid phase separation of blends quenched below the melting point. Three types of measurements were carried out in order to obtain the information : (1) kinetics of phase separation vs crystallization by using the invariant, (2) the most probable wavenumber of concentration fluctuation q(m) having the highest growth rate, (3) the apparent diffusion coefficient. The positions of the virtual UCST obtained by different methods were in good agreement with each other. In total, the LCST’s and UCST’s form a closed miscibility area for PCL/SAN blends in the temperature-copolymer composition plane. This behavior is interpreted in terms of an equation-of-state theory. TEM micrographs were taken in order to confirm the difference in morphology between the blends quenched below and above the virtual UCST. The photographs showed the familiar lamellar crystal-amorphous structure for a system crystallized above the UCST and a morphology segregated to PCL-rich and SAN-rich phases for the same blend crystallized below the UCST.