A linearized mathematical model for the pressurized water reactor (PWR) nuclear power plant dynamics simulation is presented based on conservation of energy and mass balance. The core and coolant system are treated as a lumped parameter. The delayed neutrons effect is considered. The model state variables are linearized in first order differential equations with steady state initial values. The reactor core dynamic response is investigated through transients represented in external reactivity insertion e.g.(0.001 Δk/k),10% step decrease in core flow rate and 10 o F step decrease in core inlet coolant temperature. The simulation results demonstrated the role of feedback reactivity form coolant temperature and Doppler Effect in stabilizing the core power and affecting the steady state values of core variables.