---

In this paper, the algorithms for low frequency non-linear dynamic modeling and frequency model determining of LPRE is presented. Considerations that facilitate modeling and debugging processes is also investigated. Using of defined algorithms and also presented considerations is considered for a liquid propellant engine with oxidizer and fuel tanks. Describing equations of LPE is classified as many subsystems. Simulation is done in the SIMULINK environment of MATLAB software. Each simulated subsystem show one or more physical subsystem that their interaction is determined in LPE configuration modeling results demonstrate excellent dynamic behavior of LPE. Then SISO engine model in frequency domain is outcome based on resulted non-linear model of LPE using describing function. Frequency response code is developed for derivation of engine frequency model. Adequate frequency interval and input or excited signal amplitude are selected regarding LPE operating modes. In next step, frequency model is derived by stimulation of non-linear dynamic model with sinusoidal inputs includes considered amplitudes and frequencies. This subject is done by integration and engine output obtaining and Furrier integrals calculation at time that output get to steady state. Then system gains and phases calculation is done at the various amplitudes and frequencies for obtaining describing functions models. Frequency model evaluation characterized that can provide more efficient, simple and adequate conditions for analysis of LPE dynamics.