In the practical processing conditions of poly(butylene terephthalate)/copolycarbonate (PBT/cPC) blends, crystallization starts from a partially demixed melt. To understand the crystallization mechanism in commercially important blends, we carried out time-resolved light scattering, transmission electron microscopy and small-angle X-ray scattering studies of crystallization in blends with various compositions at various crystallization temperatures. Even after crystallization, memory of demixing via the spinodal decomposition remained and crystallization seems to proceed in the PBT-rich region. PBT crystal lamellae were visualized by electron microscopy and the thickness and spacing were measured by X-ray scattering analysis. Kinetic analysis by light scattering showed a regime transition from regime II to regime I. The kinetic results were successfully interpreted by the modified Hoffman-Lauritzen theory involving a two-step diffusion mechanism characteristic of crystallization of a polymer blend, i.e. mutual diffusion for the formation of the first stem at the crystal surface and self-diffusion for the attachment of following stems in the chain.