In many applications, the break between successive missions provides an opportunity to perform maintenance under limited resources. Such maintenance policy is called selective maintenance. Traditionally, it was assumed that a component after repair may be as good as new or as bad as old. However, maintenance can bring a component in between these two extreme cases as well. This maintenance policy is called imperfect repair. Selective maintenance optimization under imperfect repair is studied in this paper. Age reduction model is used to represent imperfect repair for selective maintenance. It is suggested in  that age reduction factor depends on the maintenance cost, and a constant is used to reflect whether a component is relatively young or old. In this paper, a formulation is used for this characteristic constant which depends on the effective age of the component. It is shown that the formulation helps in establishing a relationship between age reduction factor, cost of maintenance, and effective age of the component. Also, advantage of selective maintenance with imperfect repair is shown and results are compared with the case when only minimal repair and replacement are considered as repair options.