Microphase separation and crystallization in thin films of all-conjugated diblock copolymers poly(2,5-dihexyloxy-p-phenylene)-block-(3-hexylthiophene) (PPP-b-P3HT) with various main chain lengths were investigated under solvent vapor annealing or thermal annealing. It is shown that crystallization and microphase separation coexisted during thin film forming process due to the incompatibility and crystallization of the two conjugated blocks. When the B34T66 copolymer film was annealed in chlorobenzene vapor or at high temperature, crystallization broke out the microphase-separated structure. On the other hand, upon annealing of the films of B62T38 and B75T25 with higher PPP ratio, crystallization was confined in the microphase-separated domains. The copolymer composition plays a key role in determining the crystallization and the final morphology of the thin films.