Regarding the ratios of transfer conductances to transfer susceptances as small parameters, the perturbation method is applied to construct a Lyapunov function for analysis of the transient stability of power systems. The Lyapunov function obtained here takes into account some part of the transfer conductances in the power system network. By using this Lyapunov function the transient stability of a practical power system with non-negligible transfer conductances is analysed. The results are shown in terms of the critical fault clearing time and are consistent with those of simulations