An optimal model is presented in this paper, aiming at facilitating cost effective effort allocation among software components so that a quantified system reliability goal is attained. The software reliability is defined as a function of component failure intensities, the operational profile, expected component utilizations and the stated mission time for failure-free execution. The details of building the optimal model and the steps of the solution are presented. In the process of solution, nonlinear programming theory can be used to solve the optimal model. Finally, a simple example is provided to show the feasibility of the optimal model for the allocation of software reliability and cost.