Energy gain by an electron during its motion in the field of a fundamental ${\rm TE}_{10}$ mode excited by a microwave in a rectangular waveguide is analysed. Expressions have been obtained for the fields of the mode, angle of deflection and the acceleration gradient of the electron, when it is injected along the direction of mode propagation. A 25 keV electron gains an energy of 120.44 keV when a microwave of intensity $10^{10}$\,W\,m$^{-2}$ at frequency 6 GHz is employed to excite the mode in a waveguide of width 0.03 m and height 0.02 m. The effect of the microwave frequency and width of the waveguide is to decrease the electron energy gain for a fixed microwave intensity. However, higher acceleration gradients are obtained when the electron is injected with a larger energy.