Abstarct: In today's world, the use of new and renewable energy is very important. In areas where the water flow is sufficient, and has suitable head, the capacity can be used for the construction of a hydroelectric power plant and generate electrical power. Shirin-Darreh Dam water transmission pipeline to Bojnoord also has great potential to build a hydroelectric power plant. In this study, due to the existing head and water flow, a turbine with suitable size is chosen for energy producing and then the transient flow of water in the pipeline, caused by load rejection situation, is studied and analyzed numerically. Load rejection is a special case that happens when the power plant is taken out from the electrical grid. When it happens, the turbine wicket gate starts to be closed according to the predefined plan. The most critical situation in a power plant occurs during the load rejection condition. The speed rise of the turbine and the pressure rise in the turbine and the pipeline, can cause damages to the facilities. The analysis of the transient flow, caused by the possible load rejection situation, is necessary before constructing the power plant, which is accomplished in this study. To do this, using the method of characteristics, an analytical program was developed in Visual Basic. The Hammer software was then used to verify the results of computer code. Then the pressure head graph in different parts of the pipeline and the graph of the rotating speed of the turbine were plotted, which has special importance. This kind of the flow cut off was compared to the transient flow caused by slow closing the valve at the downstream which there is not any turbine at the downstream end of the pedestal for the case when there is no turbine at the downstream of the pipeline. For these two situations, the pressure head was compared at different parts of the pipeline and the conclusion was made at the end. Finally, it was concluded that during load rejection in the in the above mentioned power plant, the maximum pressure rise happened exactly behind the turbine. In this research, it was determined that the maximum pressure rise in the turbine was about 29% of the primary pressure and also the maximum speed rise in the turbine was about 56% of the primary speed of the turbine. Also, by comparing between the two conditions of load rejection and slow closing of the valve, and observing two points, one at the end of the pipeline and the other at the middle of the pipeline, it was noticed that during the closing time, the pressure was never less than the primary pressure.