The morphology and melting behaviour of epsilon-caprolactone-styrene diblock copolymers (PCL-b-PS) quenched from the melt were investigated by small-angle X-ray scattering and differential scanning calorimetry as a function of glass transition temperature of the PS block, T-g.PS A plasticizer miscible only with the PS block, such as polystyrene oligomer or dioctyl phthalate, was added to PCL-b-PS in order to decrease T-g.PS successively (plasticizer-added PCL-b-PS). The crystallizability of the PCL block depended strongly on T-g.PS; the PCL block did not crystallize when T-g.PS was higher than the crystallization temperature T-c, while it crystallized partially when T-g.PS was lower than T-c. However, details of the morphology formed (i.e. the long spacing and lamellar thickness) were independent of TR,PS These results suggest that the mobility of PCL-b-PS influences extremely the crystallizability of the PCL block, but not the resulting morphology. The morphologies formed in the plasticizer-added PCL-b-PS were compared with those of PCL-b-PS cast from toluene or cyclohexane solution, and the characteristics of these morphologies were qualitatively discussed.