In this work, the effect of poly (phenylene oxide) (PPO) in tetrafunctional epoxy resin (4EP) was investigated in terms of cure kinetics, thermal properties, and mechanical interfacial properties of the blends. The content of PPO was varied within 0, 5, 10, 15, and 20 phr to neat 4EP. The cure kinetics of 4EP/PPO blend system are examined by near-IR and DSC measurements. And the thermal stabilities were determined by initial decomposed temperature (IDT), thermal stability factors, and integral procedural decomposition temperature (IPDT) of the blends. For the mechanical interfacial properties of the casting specimens, the fracture toughness test was performed, and their fractured surfaces were examined by SEM. As a result, the conversion (α) is indicated in high value at 5 phr of PPO and, the cure activation energy (Ea) is decreased at 5 phr PPO, due to the plasticized PPO polymer molecule in epoxy resins. The thermal stabilities were increased, which can be explained by the presence of phenyl group of PPO in intermolecular chains. Also the fracture toughness parameters (KIC, GIC) show high values at 5 phr PPO. This result is interpreted in the development of interfacial adhesion force between intermolecules of the polymer chains.