Thermal-history dependence of phase separation induced by radical polymerization of 2-chlorostyrene in the presence of polystyrene was studied by making a stepwise temperature change (jump) to a higher temperature T-2 during the phase separation. Morphological structure of poly(2-chlorostyrene)-rich domains changed from droplets to continuous domains after the temperature change if the duration at the first phase-separation temperature TI was longer than a certain value t(1)*. The morphological change to the continuous domains was accompanied by an increase of the volume of poly(2-chlorostyrene)-rich phase; The increase of the volume was observed even when temperature was jumped after the polymerization reaction finished. The minimum duration t(1)* at T-1 required for the morphological change to occur after the temperature jump increased with T-1 but scarcely depended on T-2. An explanation for the experimental results was proposed on the basis of the difference in the viscosity dependence between the phase-separation rate and the reaction rate.