A gate current variation measurement method is proposed to examine the surface roughness of metal oxide semiconductor field effect transistors (MOSFETs). This gate current variation is demonstrated on the trigate structure MOSFETs. It was found that the standard deviation of oxide-thickness is proportional to the inverse of square-root of device areas, and its slope is defined as the effective surface roughness variation. In particular, for the transistors with varying fin height, this surface roughness effect aggravates with the increasing fin height. More importantly, the gate leakage at off-state, i.e., Vg = 0 V, is strongly dependent on the gate dielectric surface roughness and dominates the drain current variations. This gate leakage may serve as a quality measure of a low power and energy efficient integrated circuit, especially for the transistor with 3-dimensional gate structure. The present results provide us better understandings on an additional source of Vth fluctuations, i.e., the surface roughness variation, in addition to the random dopant fluctuation, that we are usually not noticed. In particular, this study also provides us a simple easy-to-use method for the monitoring of oxide quality in the volume production of trigate MOSFETs.