In order to fabricate polymer field-effect transistors (PFETs) with high electrical stability under bias-stress, it is crucial to minimize the density of charge trapping sites caused by the disordered regions. Here we report PFETs with excellent electrical stability comparable to that of single-crystalline organic semiconductors by specifically controlling the molecular weight (MW) of the donor-acceptor type copolymer semiconductors, poly (didodecylquaterthiophene-alt-didodecylbithiazole). We found that MW-induced thermally structural transition from liquid-crystalline to semi-crystalline phases strongly affects the device performance (charge-carrier mobility and electrical bias-stability) as well as the nanostructures such as the molecular ordering and the morphological feature. In particular, for the polymer with a MW of 22 kDa, the transfer curves varied little (ΔVth = 3∼4 V) during a period of prolonged bias stress (about 50 000 s) under ambient conditions. This enhancement of the electrical bias-stability can be attributed to highly ordered liquid-crystalline nanostructure of copolymer semiconductors on dielectric surface via the optimization of molecular weights.