Based on the structure of strained Si/SiGe NMOSFET, a unified drain current model is presented in this paper. The model describes current characteristics from subthreshold to strong inversion as well as from the linear to the saturation operating regions with a smoothing function, and guarantees the continuities of the drain current and its derivatives.Furthermore, the model accuracy is enhanced by including carrier velocity saturation and channel length modulation effects. Comparisons between the model and the measured data show that the drain current model can describe the device characteristics well. The proposed model is useful for the design and simulation of digital and analogy circuits made of strained Si.