Energy gain by an electron during its motion in the fields of the fundamental TE10 mode excited by microwave radiation in a rectangular waveguide is analysed. Expressions have been obtained for the fields of the mode, the angle of deflection and the acceleration gradient of the electron when it is injected along the direction of mode propagation. An electron of energy 25 keV gains 120.44 keV when a microwave of intensity 1010 Wm−2 at frequency 6 GHz is employed to excite the mode in a waveguide of width 0.03 m and height 0.02 m. The effects of the microwave frequency and the width of the waveguide are to decrease the electron energy gain for fixed microwave intensity. However, higher acceleration gradients are obtained when the electron is injected with greater initial energy.