A field-effect transistor (FET) with thin films of picene has been fabricated on SiO2 gate dielectric. The FET showed p-channel enhancement-type FET characteristics with the field-effect mobility, mu, of 1.1 cm(2) V-1 s(-1) and the on-off ratio of >10(5). This excellent device performance was realized under atmospheric conditions. The p increased with an increase in temperature, and the FET performance was improved by exposure to air or O-2 for a long time. This result implies that this device is an air (O-2)-assisted FET. The FET characteristics are discussed on the basis of structural topography and the energy diagram of picene thin films.