Crystallization of poly(ethylene glycol)-poly(butadiene) (PEG-PBd) diblock copolymers and their blends with the corresponding homopolymer, where Tg of PBd blocks is much lower than Tm of PEG, was investigated with synchrotron radiation small-angle X-ray scattering (SAXS) technique and differential scanning calorimetry. It was found from SAXS results that BE69 (PEG’s fr.=0.69) was miscible with a small-molecular-weight homopolymer PBd at the total PEG contents of 30 and 50 wt% to form microphase separation structure, while it was immiscible in the blends with a high-MW PBd to macrophase-separate into block copolymer and homopolymer phases. The blends of BE31 and small-MW PEG was miscible at 50 wt% PEG. In these blends as well as the neat copolymers, crystallization from the lamellar melt destroyed the microphase structure while it was kept in the cylindrical structure.