The interpenetrating polymer networks (IPNs) were formed by unsaturated polyester resin (UPR) polymerized by free radical initiators: benzoyl peroxide (BPO) or cumene hydroperoxide (CHP) and epoxy polyester (EP), cured with acid anhydrides: tetrahydrophthalic anhydride (THPA) or maleic anhydride (MA). IPNs consisting 10, 30, 50, 70, 90 wt% of EP were prepared. The effect of the EP component in the IPNs and the type of curing agent on the cure behavior, thermal, and viscoelastic properties have been investigated. The results showed that both EP content and used curing system influenced on studied properties. As the EP content increased, the glass transition temperatures (T-g) also increased. Moreover, higher values of tan delta(max) and lower values of cross-linking density in a rubbery state (nu(e)) of IPNs containing higher EP content, probably due to plasticization effect of EP component were observed. Additionally, more heterogeneous network structure (higher values of the full-width at half-maximum (FWHM) as the EP content decreased was prepared. The thermal and viscoelastic properties of the blends cured with BPO/MA or CHP/MA system were considerably better than those cured with BPO/THPA or CHP/THPA. The higher stiffness, nu(e), T-g and lower tan delta(max) values were obtained. It was probably connected with the interactions of carbon-carbon double bonds of MA with vinyl monomer (styrene), UPR and radical initiators causing to obtain more cross-linked polymer network structure. This supposition was confirmed on basis of the cure reaction monitored by DSC. The chemical interactions between two components of the blends and epoxy hardener caused that the BPO/MA or CHP/MA cure systems influenced on the cure behavior of UPR and EP components in the IPNs. The exotherm peak temperature (T-max1) shifted to lower values compared to these in the neat UPR whilst T-max2 shifted to higher values than in the neat EP. However, the cure behavior of the UPR was not greatly affected by the presence of EP component when BPO/THPA or CHP/THPA cure systems were used due to the lack of chemical interactions between the components and their curatives.