Long-term load forecasting is an important component for power system energy management and reliable power system operation. The Grey model GM (1, 1) based on the grey system theory has been extensively used as a powerful tool for load forecasting in recent years. In this study the accuracies of two different grey models include original GM (1, 1) and modified GM (1, 1) using Fourier series have been investigated. Also, the performance of these models with ARIMA as a conventional forecasting model has been compared. Numerical results show that the modified GM (1, 1) provides better performance in model fitting and model forecasting.