In this paper, an analytic current-voltage model in the subthreshold regime for submicrometer fully depleted (FD) silicon-on-insulator (SOI) MOSFET's is presented. This model takes into account the dependence of the effective depleted charge on the drain bias and the voltage drop in the substrate region underneath the buried oxide. In addition to predicting accurate subthreshold current-voltage characteristics and subthreshold slope, this model can be used to predict important Short Channel Effects (SCE) such as the threshold voltage roll-off and Drain-Induced Barrier Lowering (DIBL), This model is verified by comparison to a two-dimensional device simulator, MEDICI. Good agreement is obtained for SOI channel lengths down to 0.25 mu m.