Abstract: By the multistage decision operator technique,this paper successfully solves the problem of optimal reliability distribution under two cases,(1)The reliability is maximized un-der condition that the total cost and loss expectation are given.(2)The total construction cost and loss expectation is minimized under predetermined reliability of the engineering system. Then the truss structure is taken as a system and is optimized based on optimal reliability dis-tribution.The concept is clear and the methot is simple,and effective.