Generation scheduling is a most important task in the operation and planning of power system. It can be on real term, short term, medium term, and long term basis as per the scheduled horizon. Scheduling of purely hydroelectric system differ from the thermal scheduling and hydrothermal scheduling problems in two aspects firstly fuel cost for the thermal part of the system can no longer be used in the objective function. Secondly water availability in seasonal reservoirs is the necessary part of such system. Many researchers suggested different solution algorithms of optimization for each type of problem and claimed better solution. This paper presents a Natural Exponential Inertia Weight Particle Swarm Optimization (NEIW_PSO) approach to determine the optimal generation schedule of hydro power plants of cascaded hydroelectric test system.