A methodology is presented to plan reuse strategies of common modules in a product family by using the concepts of function degradation, reliability, function requirement, cost and life time. Markov chain model is employed to predict function degradation and reliability. A utility model is used to evaluate the preference between used modules and new modules. An example of cascading-requirment product family illustrates the main ideas of our work. The Markov models are used effectively to predict function degradation and reliability. Utility theory is helpful to evaluate the reuse options of common modules.