The product life cycle concept suggests that a product passes through four stages of evolution: introduction, growth, maturity and decline. As the product evolves and passes through these four stages, profit is affected, and different strategies have to be employed to ensure that the product is a success within its market, therefore there is a need to define in time at which of the stages the product is at the moment. We propose ontology-based knowledge acquisition system for product life cycle stage definition. Knowledge acquisition is the transformation of knowledge from the forms in which it exists into forms that can be used in a knowledge based system. The proposed knowledge system development sequence is the following: first, concepts and their relationships are defined by an ontology. Second, the domain experts enter their knowledge of the domain area using the domain-specific knowledge acquisition tool. Finally, problem-solving techniques are used to answer questions and problems of the domain using the knowledge base.