Previous studies mostly focused the voltage characteristics of load rather than the frequency characteristics. The frequency may vary obviously in the small or weak power grid, so the frequency characteristics should be studied. The fifth-order induction motor load model and the new third-order practical induction motor load model are given. A method for load parameter estimation is proposed, i.e. estimating the key load parameters through ant colony optimization (ACO). In a simulation case, the identification results of two load models are compared with and without considering frequency characteristics. Finally, the measured data under a disturbance in Xinjiang Power Grid are used to validate the necessity of considering frequency characteristics. It is indicated that the load parameter estimation and simulation results with considering frequency characteristics are better than that without considering frequency characteristics, and the fifth-order induction motor load model is better than the new third-order practical induction motor load model.