Abstract: The renewable energy sources such as wind power and solar energy are widely connected to the current power grid. The proportion of the renewable energy to the entire energy sources in the power system is increasing. Since wind power and solar energy are intermittent, which have more uncertain factors, power grid operation is being impacted by installation and generation of the renewable energy sources. Energy balance or power balance is of great importance to power system operation. However, the uncertainties of renewable energy bring issues to power system power balance and dispatch. Severe power imbalance, which causes the huge frequency deviation, will lead to the stability of power system. Therefore, it is very important to deal with the uncertainties of renewable energy. The existing deterministic approaches are not sufficient to deal with the uncertain factors. This paper analyzes the effect of renewable energy uncertainties in power grid operation based on the fuzzy theory and probabilistic methods. The proper approaches to handle renewable energy uncertainty in power grid operation are recommended.