Designing a reliable system is difficult and costly, due to the design process to be followed and test requirements. It is important to consider reliability in very early design phases, and it is essential to select a baseline design with attainable high reliability. Therefore, in early phases, it is required to predict reliability and evaluate possible improvements for the reliability of the design alternatives in a fast manner to make necessary design decisions. In this study, a solid rocket motor is considered as a case study, and it is aimed to determine important parameters that affect the solid rocket motor reliability, to assess ballistic performance and casing structural reliability, and to find a new design point to evaluate possible improvement range for its reliability. The variations in dimensions and material properties, which are obtained from the previous data, are considered as the sources of failures and the limit states for acceleration, and the total impulse and the maximum stress in the casing are approximated by the response surface method. Monte Carlo simulation is used with the response surface functions to assess the failure probability and distributions of the rocket motor performance parameters. By considering the effect of the input parameters and distribution functions of the performance parameters, a new design point is proposed to decrease the total probability of failure.