In this paper, based on the two-dimensional (2D) Possion's equation, an analytical model of threshold voltage, which is applied to a sub-100nm strained-Si/SiGe nMOSFET, is pro- posed. The secondary effects induced by reducing size such as short-channel effects, quantum mechanical effects are also taken into consideration in order to ensure the accuracy of the model. Then the evidence for the validity of our model is derived from the comparison between analytical results and the simulation data from the 2D device simulator ISE. Finally, the influence of conventional arts in sub-100 nm device fabrication on threshold voltage is also discussed. The proposed model can also be easily used for reasonable analysis and design of sub-100nm strained-Si/SiGe nMOSFET.