The crystallization and microphase separation of compatible mixtures of tetrahydrofuranmethyl methacrylate diblock copolymers (PTHF-b-PMMA) with a tetrahydrofuran homopolymer (PTHF) were studied by means of synchrotron small-angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC). The crystallization of PTHF microphases in the blend consisting of a PTHF-b-PMMA diblock copolymer and a PTHF homopolymer showed almost no effect on the original phase-separated structures in the amorphous state of the blend before crystallization including, for example, the PMMA interdomain distance. In the blends with PMMA lamellar or cylindrical microdomains, the crystallization temperature on cooling and the crystallinity as well as the melting point on subsequent heating of the PTHF microphase were very sensitive to the PTHF lamellar thickness or the distance between the surfaces of two neighboring PMMA cylinders. After crystallization, the blends with an alternating lamellar structure showed an appreciable intensity increase in the higher order scattering peaks, suggesting that the lamellar packing had become more regular. The separation distance between two PMMA cylinders (or spheres) had a great effect on the crystalline structure of the PTHF matrix microphase. When the separation distance between two PMMA cylinders was about 2 times larger than the long period of the neat PTHF homopolymer, a nearly ordered microcrystal packing could be formed in the PTHF matrix microphase. In the blends with 30 wt % or less copolymer and with larger separation distances, the microcrystals could have a very ordered packing, exhibiting sharp X-ray peaks similar to that of the PTHF semicrystalline homopolymer. The PTHF matrix microphase also showed an increase in the long period with increasing copolymer weight fraction. However, this increase in the long period did not result directly from PMMA microdomains. The relationship between the long period and the crystallization temperature was different for blends with different amounts of copolymer.