Organic magnetoresistance (OMAR) of single-crystalline (SC) pentacene (C22H14) and perfluoropentacene (C22F 14) was studied using field-effect transistor structures. The gate voltage effect showed that the OMAR originates from photo-induced current and requires both electrons and holes in the transport channel. The temperature dependence showed the maximum magnetoresistance (MR) ratio up to -6% under light irradiation at approximately 200 K and magnetic field of 80 mT. The charge carrier mobility and the exciton diffusion length were not important factors to determine the MR ratios. The interaction between triplet excitons and traps was thought to govern the OMAR behaviors.