Based on the theory of hyperelasticity, we propose a method for calculating large elastic deformations of composite interphase layer, which consists of macromolecules constrained at their one end to active centers of the surface of filler. Taking into account surface effects, we derive the analytical relationships between the components of the stress tensor, on the one hand, and the relative elongation and the molecular and structural parameters of macroscopic molecules of interphase layer, on the other hand.