Two types of additives, phenoxy thermoplastic (PKHH) and core-shell rubber (CSRs) particles were studied at first separately to improve the toughness of highly cross-linked epoxy resin. The influence of the added amount as well as the mixing time have been investigated. 10 wt% of additive are selected as a compromise in order to obtain a low viscosity blend adapted to composite resin transfer moulding process, while keeping the mechanical and thermomechanical properties. The combination of both materials revealed a synergistic phenomenon. The results show that the critical energy release rate (GIC) of the hybrid (5 wt% PKHH and 5 wt% CSRs) modified epoxy system increased by more than ~ 210 % compared to the unmodified epoxy system while the individually modified epoxy systems (10 wt% PKHH or 10 wt% CSR) reveal an increase of about 100% respectively. The resulting morphology of the fracture surfaces shows the presence of propagation cracks deviated/arrested by the CSRs particles surrounding the phenoxy nodules dispersed in epoxy matrix as main toughening mechanisms.