In theory, private power plants have incentives to withhold outputs to exercise market power. on the other hand, they also have incentives to operate power plants efficiently to minimize operation costs. Therefore, the ultimate answer for this question is an empirical subject. Using two types of unbalanced panel data, we estimate input demand function to examine whether private power plants used fuels efficiently relative to a set of plants that remained under government control or experienced recent ownership changes. Our estimation results suggest that private power plants used fuels efficiently about 6.3%～10% relative to a set of other power plants. In addition, private power units operated about 14% efficiently compared to public power plants.