Abstract: The development of middle and long term load forecasting plays an important role in power grid planning, but the key issue is to build mathematical models. The grey load forecasting model based on modified Markov is built in this paper, in which the grey forecasting model is used to forecast future load and forecasting results is modified by Markov forecasting method to improve its accuracy. In the end, grey Markov model is built to forecast load demand for certain city from 2006 to 2008, which verify the high accuracy of application of grey Markov model to long term load forecasting through comparison with that of actual load.