In this paper we suggested a performance evaluation model for future weapon systems. Weapon Performance Index(WPI) model transform the characteristics of alternatives as indices. We can easily obtain WPIs of alternatives with the model. The highest WPI recommended as the best solution. The performance elements in hierachy for future weapon systems are determined by systems engineering procedure. Priorities in hierachy can be determined through survey of experts engineering procedure. Priorities in hierachy can be determined through survey of experts and statistical analysis. Utility function is formulated as a probability model and utility score is predicted on the basis of historical data about the same category of weapon systems in the world. WPI is calculate from sum of product of priorities and utility scores. The model can be applied to trade-off analysis, cost and effectiveness analysis, war game model.