In a certain class of strained n-channel metal-oxide-semiconductor field effect transistor (MOSFET) with silicon-carbon (Si:C) as a stressor in its source/drain, it serves as good candidate for high mobility and drain current device. However, its drain current (Id) fluctuation and the threshold voltage (Vth) fluctuation, have not been clarified. This paper reports a systematic method to analyze the sources of the above two different fluctuations represented by σId and σVth, respectively. The dominant sources of the σId and σV th have been clarified on experimental n-channel Si:C source/drain FETs. The Id fluctuation relies on the dopant fluctuation or the mobility factors related to the conductions at various biases. Results show that the Id fluctuation at low field or low gate bias, i.e., near the threshold, is dominated by the RDF (Random Dopant Fluctuation) effect, while at high field, it is dominated by the channel conduction and scattering events which can be adequately described by the changes of mobility. The abnormal increase in the RDF effect in the Si:C was induced by the carbon out-diffusion from the drain into the channel. A dopant profiling technique has been developed to validate the out-diffusion effect.