In the present paper, the damage of fiber-reinforced composite laminates is considered with the aim to examine the change of their mechanical properties. The crucial issue for theoretical analysis is to construct constitutive relations, which talce into account the development of damage, along with stress and strain. The paper is mainly focused on this problem. In order to derive an adequate description, the author employs an approach based on polynomial invariants functions and invariants integrity basis by Adkins.