We investigated the bias voltage effect on the electrical properties of n-type polymer field-effect transistors (FETs) with dual gate electrodes. The electrical characteristics of such polymer FETs were measured as a function of the sweeping voltage at one gate electrode when the other gate electrode was floated or biased. The hysteresis was negligible when the top gate voltage was swept while the bottom gate was biased. The threshold voltage decreases and the mobility increases with increasing the bottom gate bias due to the increase of mobile charges.