The influence of ultraviolet (UV) irradiation on carrier conduction and emission characteristic of the top-gate-type ambipolar organic field-effect transistors (OFETs) utilizing crystallized poly(9,9-dioctylfluorene) (F8) films were investigated. For a device with irradiated UV light, only p-type conduction was exhibited and the hole field-effect mobility was almost the same as that without UV irradiation. As a result, the EL emission was not observed owing to the decrease of n-type conduction after UV irradiation. It is suggested that the quenching center which is generated in F8 film by UV irradiation is greatly related to the electron trap site of OFETs.