A series of thermal cross-linkable poly(3-hexylthiophene)-b-poly[3-(6-hydroxy)hexylthiophene] (P3HT-b-P3HHT) diblock copolymers (BCPs) with different block ratios were synthesized to explore the effect of cross-linking on crystallization and microphase morphology of polymer films. The obtained BCPs formed microphase separation structure with a lamellar packing due to the different polarity of the two blocks. Though, after thermal annealing at 200 degrees C, the cross-linking of P3HHT block damaged the microphase separation structure and the roughness of films increased, the crystalline structure still remained "edge on", and the degree of crystallization greatly improved due to the rearrangement of the un-cross-linked parts. Based on the weight loss of the cross-linking reaction, the degree of cross-linking was calculated, and the ductility of film was characterized by the crack onset strain. Then a quantitative correlation between the ductility of film and the degree of cross-linking was established. It proved that cross-linking during thermal annealing at 200 degrees C can both improve the ductility of film and the degree of crystallization, which is a promising way to develop flexible electronic devices.