An effect, called random trap fluctuation (RTF), is proposed to study the threshold voltage (Vth) fluctuation of metal oxide semiconductor field effect transistors (MOSFETs) under Fowler-Nordeim (FN) or hot carrier (HC) stress condition. Experiments have been demonstrated on n-channel MOSFETs, and it was found that not only the random dopant fluctuation (RDF) but also the stress-induced traps vary the Vth fluctuation. More importantly, the stress-induced trap barrier determines the Vth fluctuation. For devices after FN stress, Vth fluctuation is enhanced since the trap barrier regulates the transporting carriers. For devices after HC stress, V th fluctuation is supressed since the carriers are backscattered into the channel by the trap barrier and fewer carriers with higher energy pass through the barrier. These results provide us a clear understanding on another source of Vth fluctuations in addition to the RDF as devices are further scaled.