As the gate length of MOSFET devices shrinks down below 100 nm, the fluctuation of major devices parameter, namely, threshold voltage (V TH), subthreshold swing, drain current (ID) and subthreshold leakage current due to influences of processes variations becomes a serious problem. Random dopant fluctuation is one of the problems. In this work, we theoretically examine the fluctuation effects of random dopant on the threshold voltage and drain current variation in single-gate (MOSFET), SOI and double-gate MOSFETs. In the numerical simulation of the threshold voltage variation, the drift-diffusion and density gradient models are considered to describe transport phenomena with quantum effects of devices. Random dopant induces drain current and threshold voltage lowering. The reduction becomes larger with channel length is scaling down. From the results, VTH can be controlled by thin channel DG-MOSFET and SOI devices. However, thin channel may reduce ID. Thus, structure of devices needed to be optimized. The fluctuation of device characteristics caused by random dopant cannot be neglected. From the fabrication point of view, we concluded that the random dopant fluctuation of device characteristics could be controlled in the design of SOI and DG-MOSFET.