Herein is demonstrated that the polymer chain ends of polymer gate- dielectrics (PGDs) in organic field-effect transistors (OFETs) can trap charges; the bias-stress stability is reduced without changes in the mobilities of the transistor devices as well as the morphologies of the organic semiconductors. The bias-stress stabilities of OFETs using PGD with various molecular weights (MWs) are investigated. Under bias stress in ambient air, the drain current decay and the threshold voltage shift are found to increase as the MW of the PGD decreases (MW effect). This MW effect is caused by the variation in the density of polymer chain ends in the PGDs with MW: the free volumes at the polymer chain ends act as charge-trap sites, resulting in drain current decay during bias stress. The free volumes at polymer chain ends are sufficiently large to allow the residence of water molecules, the presence of which significantly increases the density of charge-trap sites. In contrast, polymer chain ends without trapped water molecules do not allow charge trapping and so bias-stress stability is independent of the MW of the PGD. It is also found that the hydrophilicity/hydrophobicity of the chain ends of the PGD can affect bias-stress stability; carboxyl-terminated polystyrene exhibits a much higher trap density and lower bias-stress stability than hydrogen-terminated polystyrene when these devices are exposed to humid nitrogen.