A new tool for designing a solid propellant rocket motor casing made of composite materials is obtained by merging the genetic algorithm optimization method with the finite element method. The genetic algorithm optimization method is used to determine the best design parameters of the laminate to minimize the weight of the rocket motor casing. The ANSYS finite element code is used to analyze the overstressing strength of the rocket motor casing. Tsai-Wu failure criterion is used to assess the first ply failure and an overstressing load level factor is introduced to describe the failure strength. Practical design examples, of composite rocket motor casings, are investigated using the present design tool.