All-conjugated rod-rod block copolymers (BCPs) have gained immense interest over the past few years because they combine fascinating self-assembly properties of BCPs with the optical and electronic properties of conjugated polymers. Based on it, an all-conjugated rod-rod BCPs, poly(3-hexylselenophene)-b-poly[3-(6-hydroxyl)hexylthiophene] (P3HS-b-P3HHT) with hydroxyl groups as side substitution groups was synthesized via the Grignard metathesis (GRIM) method. The introduction of side hydroxyl groups was designed to endow different polarity between P3HS and P3HHT blocks and enrich the solution structures of P3HS-b-P3HHT. During thermal annealing, the cross-linking of hydroxyl groups was also utilized to improve the thermal stability of poly(3-hexylthiophene) (P3HT)-based organic field-effect transistors (OFETs) when blended with a certain amount of P3HS-b-P3HHT. On one hand, the use of mixed solvents provided an effective way to control the self-assembly behavior of P3HS-b-P3HHT. Depending on the mixed solvent ratio (i.e., chloroform/pyridine or methanol/pyridine), the rod-rod interaction of the copolymer chains was controlled, yielding a series of nanostructures such as nanoribbons, nanofibers, and nanospheres. Detailed morphologies and the corresponding photophysical behavior of different nanostructures were characterized by transmission electron microscope and UV-vis absorption spectra. The conformations of the P3HS and P3HHT chains in the solutions influenced their photophysical properties greatly. On the other hand, based on the thermal cross-linkable properties of hydroxyl groups, a certain amount of P3HS-b-P3HHT was mixed with P3HT homopolymer to fabricate P3HS-b-P3HHT/P3HT OFETs. For control samples, the charge carrier mobility of pure P3HT-based OFETs was improved with the increased annealed temperatures up to 170℃, then decreased significantly when the temperature further increased to 200℃. While overall, the charge carrier mobilities of P3HS-b-P3HHT/P3HT OFETs were lower than those of pure P3HT-based OFETs, they were improved with the increased temperature to 200℃. It was found the P3HS-b-P3HHT(10%)/P3HT OFETs exhibited the charge carrier mobility of 0.040 cm2·V-1·s-1 after annealing at 200℃ for 1 h, which was higher than P3HT OFETs (0.025 cm2·V-1·s-1) under the same experimental condition. It was due to the cross-linking of hydroxyl groups in P3HS-b-P3HHT retain the crystallization structures of P3HT, thus improved the thermal stability of OFETs. Overall, this work demonstrates a new polythiophene-polyselenophene BCP with controlled nanostructures by solvent blending and promising application in OFETs to improve their thermal stability.