For a binary blend of poly(epsilon-caprolactone) (PCL) and poly(styrene-co-acryronitrile) (SAN) containing 27.5 wt% of acrylonitrile, the lower critical solution temperature (LCST) type of phase boundary was determined by thermo-optical analysis. A critical mixture, 80/20 PCL/SAN blend, initially underwent spinodal decomposition (SD) above the LCST and then crystallized at a lower temperature (40 degrees C). Isothermal crystallization was analyzed by differential scanning calorimetry. The longer the SD duration the higher the rate of crystallization An optimum SD time yielding the highest degree of crystallization was found to exist at a relatively early stage of SD (ca. 5 min). The blend via the optimum time of SD showed the highest melting point. In the blend, a very long and straight crystal lamella was observed by transmission electron microscopy. Thus, the effect of the degree of demixing on the succeeding crystallization was revealed.