The electrical characteristics of field‐effect transistors using solution cast regioregular poly(3‐hexylthiophene) are discussed. We demonstrate that both high field‐effect mobilities (ca. 0.045 cm2/V s in the accumulation mode and 0.01 cm2/V s in the depletion mode), and relatively high on/off current ratios (greater than 103) can be achieved. We find that the film quality and field‐effect mobility are strongly dependent on the choice of solvents. In addition, treating a film with ammonia or heating to 100 °C under N2 can increase the on/off ratio without decreasing the mobility.