The high temperature characteristics of 28 nm ultra-thin-body fully depleted (FD) SOI MOSFET with low threshold voltage(LVT)structure were studied. The device was tested at 300 ℃, and parameters were compared between FD-SOI and partially depleted (PD) SOI. Combined with theoretical analysis, it was proved that ultra-thin-body FD-SOI had lower threshold voltage drift rate and sub-threshold slope than PD-SOI at high temperature. When operating at 300 ℃, the parameters of SOI MOSFET degrade, the threshold voltage decreased, the leakage current increased, and the gate’s ability to control the channel current was greatly reduced. The design of the ultra-thin body FD-SOI allowed for more stable high temperature performance of the device, thus the operating temperature of the circuit was increased to 300 ℃.