An novel micromachined uncooled infrared detector based on temperature dependence of drain current of MOSFET is proposed. It utilizes a MOSFET fabricated on SOI substrate as thermosensitive element. An IR resonance cavity formed on passivation layer of MOSFET improves IR(8～14μm) absorptivity and a microbridge fabrication using micromachining technique provides a low thermal conductance. The temperature dependence of drain current of MOSFET, IR detection principle and device's structure are presented. The performance of IR detector based on numerical analysis and finite element analysis (FEA) is estimated that the detectivity (D*) can reach 10⁹～10¹⁰cmHz^1/2W^-1.