Here we report the effect of thermal annealing time on the performance of ambipolar organic light-emitting transistors (OLETs) made using conjugated polymer blends. Regioregular poly(3-hexylthiophene) (P3HT) and poly(9,9-dioctylfluorenyl-2,3-diyl-co-1,4-benzo-2,1',3-thiadiazole) (F8BT) were chosen as a p-type and a n-type component, respectively. As a gate insulator, poly(4,4'-oxydiphenylene-pyromellitimide) (PMDA-ODA PI) was employed due to its high solvent resistance and thermal stability. Results showed that the present OLETs exhibited ambipolar characteristics even after thermal annealing. All devices showed almost identical field-effect mobility for both holes and electrons. The highest field-effect mobility was achieved for the OLET annealed at 130 degrees C for 60 min, which was assigned to the improved polymer-metal contact by thermal annealing leading to better charge injection.