A systematic method to enhance the efficiency of energy transfer in a Smith–Purcell free‐electron laser composed of a metallic grating and a relativistic electron beam is proposed. To maintain the synchronism between an electron beam and a slow electromagnetic wave propagated along a metallic grating, either one of the three grating parameters is adiabatically changed in the direction of wave propagation in accordance with the decrease in the drift velocity of the electron beam. The numerical simulation, which takes into account the reflection or scattering due to discontinuities in the grating parameters, demonstrates sufficient efficiency enhancement in energy transfer. ©1995 American Institute of Physics.