The compatibility and crystallization of tetrahydrofuran-methyl methacrylate diblock copolymer (PTHF-b-PMMA)/tetrahydrofuran homopolymer (PTHF) blends were studied. Our results showed that the crystallization and morphology of compatible PTHF-b-PMMA/PTHF blends are very different from those of incompatible blends. The compatibility of the blends was successfully and very conveniently determined by analysing their crystallization behaviour, and the result is in good agreement with the compatibility determined by direct observation under an optical microscope. Some very unusual crystalline morphologies were discovered for the compatible PTHF-b-PMMA/PTHF blends resulting from the effect of PMMA microdomains in the blends.