We reported a synthesis of an ultrathin FeS₂ thin film via thermal sulfurization of an iron thin film and its fabrication process for field-effect transistors. Reaction time was found to be essential for the growth of the crystallized FeS₂ thin film. The thickness of Fe increased from 10 nm to 35 nm after the reaction. The FeS₂ thin film has a resistivity of 0.345 Ω cm and a Hall mobility of 7.1 cm² V⁻¹ s⁻¹, with a carrier concentration of 2.89 × 10¹⁸ cm⁻³. The fabricated FeS₂ transistors have a reported highest current on/off ratio of 3.94 × 10⁴ and I_on of 0.117 mA. Moreover, the FeS₂ based transistor not only broader the applications of pyrite, but also provides a platform for investigation of FeS₂ materials. Temperature-dependent electrical transport measurements confirmed the rich intrinsic defect states in the FeS₂ thin film, which indicates that reducing intrinsic defect might be the key issue to further improve the device performance.