The threshold voltage fluctuation caused by random distribution of discrete oxide charges is studied using three-dimensional (3D) simulations. It is found that the effect of discrete oxide charges on the terminal current variation depends on their lateral positions in the channel and, for the same number fluctuations of discrete charges, a large threshold voltage fluctuation is introduced in a device with deeper source/drain junction depth (X-j). The effect of 3D current flow is found to give negligible effect on the threshold voltage fluctuation. Finally, the effects of oxide charges and dopants on the threshold voltage fluctuation are compared.