The effect of crystallization temperature on the micellar morphology of PCL-b-PEO block copolymers in water has been studied. It is found that the micellar morphology of PCLnPEO44 and PCLnPEO113 changes with crystallization temperature in different ways because of two competitive factors: perfection of the PCL crystals in the core and deformation of the soluble PEO block. For PCLnPEO44, perfection of the PCL crystals dominates the micellar morphology and lamellar micelles are formed at a higher crystallization temperature. For PCLnPEO113 the micellar morphology is mainly determined by the tethering density and spherical micelles or cylindrical micelles with a larger length/diameter ratio are formed at a higher crystallization temperature because of the larger tethering density.