Determination of the grain geometry is an important and critical step in the design of solid propellant rocket motors. Because, the performance of the rocket is greatly effect by the burning surface area of the solid propellant. The performance prediction of the solid rocket motor can be achieved easily if the burn back steps of the grain are known. In this study, grain burn back analysis for 3-D star grain geometries was investigated. The method used was solid modeling (ANSYS? - DM, PRO-E and GAMBIT) of the propellant grain for predefined intervals of burn back. In this method, the initial grain geometry was modeled parametrically using ANSYS software. For every burn step, the parameters were adapted and the new grain geometry was modeled, for analysis three cases of grain geometries was considered during the combustion. By analyzing these geometries, burn area change of the grain geometry was obtained. Using this data and internal ballistic parameters, the performance of the solid propellant rocket motor in terms of motor pressure was achieved.