The object of research is the deformation characteristics of the hardened polyester-acrylate resin grade MGF-9, which is a promising basis for polymer composite materials. A curing system consisting of PKOH-2 cyclohexanone peroxide and OK-1 cobalt octoate was used. Method. The study was carried out by the method of a physical experiment using the Shimadzu Autograph AG-X Series universal testing machine in various loading and unloading modes, including a cyclic mode for taking a tension-compression diagram. The mechanical hysteresis was studied when the samples were deformed at a rate of 1 to 10 mm/min. Hysteresis losses were determined depending on the number of cycles, stress, and strain rate. The main factor determining the number of losses is the maximum deformation in the cycle. Results. The complex nature of the change in the effective modulus was revealed, which indicates a change in the molecular structure of the material during deformation and, accordingly, a change in the molecular mechanism of this process. The greatest changes occur in the first cycle, so the preliminary deformation leads to the stabilization of the state of the material. Deformation of the cured resin to varying degrees and in different modes leads to significant changes in its properties due to changes in its molecular structure. The dependences of the effective (conditional) modulus on the degree of deformation during the loading of the sample and its unloading differ significantly. The obtained results on physical and mechanical properties are suitable for use in the design of products for various purposes.