With the aid of particle simulation, we discuss efficiency enhancement in a single-pass Cherenkov laser by varying the permittivity of a dielectric sheet loaded on a parallel plate waveguide. For the analysis of the problem, a two-dimensional model for the Cherenkov laser is considered which is composed of a planar relativistic electron beam and a parallel plate waveguide, one plate of which is loaded with a dielectric sheet. In order to maintain the synchronism between an electron beam and an electromagnetic wave, the permittivity of the dielectric is properly varied in accordance with the variation in the average velocity of the electron beam. The result of numerical simulation shows that the efficiency of energy transfer from the electron beam to the electromagnetic wave is greatly enhanced by properly varying the permittivity of the dielectric sheet. ©1997 American Institute of Physics.