Despite significant advances in double-crystalline coil-coil block copolymers (BCPs), investigations into doublecrystalline all-conjugated rod-rod BCPs have been comparatively fewer and are limited in scope. Moreover, the ability to control the crystalline structures of all-conjugated BCPs may endow the materials and devices with enhanced optoelectronic properties over the two respective constituents. Herein, we report the synthesis of a series of poly(3-hexylthiophene)-block-poly(3-butylselenophene) (P3HT-b-P3BS) BCPs with tunable block ratios and investigate the effects of block ratio and thermal annealing process on their crystallization and microphase-separated structures. These rod-rod BCPs exhibit a sole P3HT crystallization (P3HT/P3BS = 63:37) or individual P3HT and P3BS crystallization (P3HT/P3BS = 55:45 and 42:58) in as-cast thin films, influenced by the block ratio of P3HT/P3BS. Interestingly, upon 200 degrees C-annealing (i.e., annealed at the temperature below the melting points of P3HT and P3BS form I blocks), P3HT-b-P3BS (P3HT/P3BS = 63:37) remains the sole P3HT crystallization, while P3HT-b-P3BS (P3HT/P3BS = 55:45 and 42:58) transforms from two individual P3HT and P3BS crystal domains into cocrystals, accompanied by the phase transition of P3BS block from form II to I. Remarkably, after a higher thermal annealing at 230 degrees C (i.e., close to the melting point of P3HT block yet below the melting point of P3BS form I block), the cocrystalline structures originally existing in P3HT-bP3BS (P3HT/P3BS = 55:45 and 42:58) at the 200 degrees C-annealing process do not form, and they reverse back to individual P3HT and P3BS form I crystals. Finally, the relationship between various structures of P3HT-b-P3BS and the resulting charge mobilities is clarified. This study provides an insight into the interplay between microphase separation of P3HT-b-P3BS and crystallization of both P3HT and P3BS blocks tailored by the block ratio and thermal annealing temperature and correlates their different structures with the charge transport properties.