The crystalline and amorphous areas in poly(tetrahydrofuran) (PTHF) domains in a poly(tetrahydrofuran-b-isoprene) (Crystalline-amorphous, PTHF = 52 wt %) film were distinguished by transmission electron microscopy and the relation between the crystallization and domain shape was clarified. The ratio of PTHF domain containing amorphous area was low in the case of the fewer interfaces of the small curvature formed by poly(isoprene) domain. The PTHF crystallization was closely related to the interface shape.