One of the common problems faced by many electric utilities concernes with the uncertainty from both load forecast error and generating unit unavailability. This uncertainty might lead to uneconomic operation if it is not managed properly in the planning stage. This paper explicitly demonstrates how to include the uncertainties to obtain the best operating strategy for any power systems. The uncertainty of the load forecast is handled using decision analysis method, meanwhile the uncertainty of the generating unit is approached by inclusion of risk cost to the total cost. In addition, three spinning reserve strategies based on deterministic criteria are incorporated in the development of scenario. Meanwhile, Mixed Integer Linear Programming method is utilized to generate unit commitment decision in each created scenario. The best strategy which gives the minimum total cost is selected among the developed scenarios. The flowchart of the proposed method is shown in Fig. 1.The proposed method has been implemented to solve a modified IEEE 24-bus system comprising 26 generating units. Three spinning reserve strategies based on deterministic criteria are set for each hour at 8%, 10%, and 12% of the load demand, representing low, medium and high spinning reserve strategy respectively. Meanwhile, the demand uncertainty is modeled by low, medium, and high Fig. 1. Flowchart of the proposed method load level with its associated probability. The load forecast uncertainty is represented by the standard deviation (SD), i.e. x% of the expected values. Sensitivity analysis with respect to the number of unit, expected unserved energy (EUE) price, standard deviation of load forecast, and probability of load level is reported.In the sensitivity analysis of number of unit, the IEEE test system is duplicated to be 26-unit, 78-unit and 130-unit system representing small, medium, and large-scale system. The load forecast error represented by SD is assumed to be 1% of the forecast value in the first hour and linearly increased to 4% in the last hour. The EUE is priced at 2000 $/MWh. The results shows that the generation cost for each scenario is a litlle bit different whereas the risk cost is rather different from each other.The impact of the load forecast error is analyzed through the variation of standard deviation (SD) values. In this analysis, it is supposed that the SD is assumed to be 1% of the forecasted value in the first hour, and is treated as either a constant value for the whole lead time of 24 hours or linearly increased to 2-4% in the last hour. The results show that with higher forecast error, i.e. higher SD, the total cost obtaied from the best strategy tends to decrease. However, as the system becomes larger, lower reserve strategy tends to be more appropiated, since the risk cost contribution to the total cost is relatively small. EUE price is varied to verify its impact on the operating strategy. It can be noticed that if the risk cost is neglected from our consideration, i.e. EUE price is set at zero, the lower spin...