The gate length dependence of PD SOI NMOS device on total dose irradiation is investigated, which is exposed to 60Co gamma ray at a dose rate of 50 rad(Si)/s. The result shows that the transistor with shorter gate length shows larger radiation-induced interface trap density, which leads to the maximum transconductance degradation. The local floating body effect induces the output characteristic variation of irradiated MOSFET with gate length. After irradiation, the breakdown voltage of short channel SOI device decreases. Due to the buried oxide, the radiation-induced degradation of short channel SOI device is much serious compared with that of long channel SOI device.