The water hammer action often bappens in pumping stations where several centrifugal or axial flow pumps having different design points and different characteristics are installed in parallel, and operated partially or totally depending on the demand of water, in order to maintain better efficiency of the plant. This paper deals with the analysis of water hammering action in parallel operation of different pumps. As one example for this analysis the parallel operation of two pumps is chosen, and hydraulic transients of each pump are calculated for two cases, where the discharge valves are closed or not after the power failure. By these calculations the following result has been confirmed : When power fails suddenly in parallel operation of the two different pumps, the discharge of one pump, of which the speed falls faster than the speed drop of another pump, does not decrease but increases for a short time after power failure.