This paper investigates the valuation of a replacement project of nuclear power plants under the deregulated electricity market. The replacement project consists of two components: the decision to decommission an existing plant and the decision to construct a new plant. In the replacement project, the decommissioning decision should be made considering not only the profitability of the existing plant but also the profitability and costs of the construction of the new plant. Real options theory is used to determine the optimal timing of the decommissioning and construction. In order to examine the effect of decommissioning time and decision making one, we consider a time-lag for these decision making times. We show the dependence of the replacement project value on uncertainty and time-lag. (C) 2009 Elsevier Ltd. All rights reserved.