Portfolio selection of new product development projects is one of the most important decisions in an enterprise that impact future business profits, competitiveness and survival. Ensuring reliability in a new product is costly but it increases customer satisfaction and reduces the potential warranty cost, contributing to product success. This paper aims to develop an approach for designing decision support system of selecting portfolio of new product development projects, taking into account the aspect of ensuring the desired reliability of products. A portfolio selection problem is formulated in terms of a constraint satisfaction problem that is a pertinent framework for designing a knowledge base. A set of admissible solutions referring to the new product alternatives is obtained with the use of constraint logic programming. The proposed approach is dedicated for enterprises that modernise existing products to develop new products.