There are several methods for starting a complex mechanical system, for example a liquid propellant engine. One of common methods is the use of solid propellant gas generator that is named solid propellant starter. In this method, a solid propellant motor is used for gas generating and leading it towards power generation turbine. The turbine as an active element, rotates one or several pumps for providing propellants with suitable head and rate for consumers such as liquid propellant gas generator and combustion chamber. After moving of pumps and reaching to nominal conditions, the start system stops. Therefore in order to suitable and optimized designing of starter, the essential parameter is taking into account the downstream resistance of system. In a complex system such as a liquid propellant engine, the start system is one of the main and important components of engine and its operation affects directly on the other components of engine. Therefore the optimized designing of it, has special importance. On the one hand, the selection of solid propellant geometry (grain) that exist in this system, is one of main parameters and the most important function in the process of start system designing, because the geometry of solid propellant is determinant of burning area and consequently the burning pattern of solid propellant. Therefore it is an important factor in determining the performance of starting system. In this paper, the alterations of pressure and thrust of a starter with different solid propellant grains have been simulated and have been compared with experimental results. The very good agreement of theoretical and experimental results indicates that the accuracy of simulation process is excellent.