Effect of thermal annealing on the performance of flexible OFETs was investigated. Annealing at low temperatures up to 50 °C enhanced the molecular ordering and granular arrangement leading to improved device performance, however, a permanent degradation was induced at high temperature annealing due to loss in crystallinity and crack generation. An annealed device exhibited a larger decay in drain current than a pristine device under constant bias stress. Under combined effects of mechanical and thermal stress, effect of thermal stress was found to be dominant at higher annealing temperatures, resulting in monotonic performance degradation. © 2017 IEEE.