Abstarct: The use of pumps lixnked in parallel or series in large scale pipe systems is usually inevitable, to meet the required head and discharge. Transient flow occurs following a pump failure in a pump group as a result of variations in the flow rate. The investigation of the pressure variations following this pump failure excitation is of great interest for pump station design purposes. The purpose of this thesis is investigation about unsteady flow regime due to different patterns of pump shut-down: one after another with a delay between and sudden failure of the whole pump group and also offer the best algorithm for the shutting down the pumps when they are connected in parallel or series to achieve the at least maximum pressure head. To investigate the unsteady flow regime in this case, examined almost all possible scenarios for switching off the pumping system to reduce the maximum pressure and increase minimum pressure head that input to system of piping. Simulating the flow in the steady and unsteady states, the governing equations include momentum and continuity of the fluid as well as pump relations. Method of Characteristics (MOC) is used to solve the transient flow regime. It was combined with the pump relations being the head balance equation across the pump and its rotational speed change relation at any instant. The validity of the homogenous relations includes the fundamental assumption allowing for seeking the pump characteristics at various speeds after shut down. From the results one can find that the intermittent shut-down as opposed to suddenly switching off the whole pump group produces much less waterhammer pressures. And also in the intermittent shut-down with different pump characteristics, it is suggested to firstly switch off the most powerful pump, and then the rest which are weaker. And on the other hand, the interval between pump shut-down is one of the factors that influence on the critical pressures. Appropriate interpretation about the transition results have been included.