Power system is a typical nonlinear system and voltage stability is an important subject of power system stability. Lyapunov energy function is an important tool for the determination of stability for nonlinear systems without the need to find exact solution. In this paper, the voltage stability of a single machine infinite bus (SMIB) system is analyzed using Lyapunov's energy function obtained by variable gradient method. Various energy curves for different equilibrium points based on the variation in load angles are obtained which clearly reflects the change in energy content of the system and load voltage in the system.