The influences of different amounts of propyl ester phosphazene (FR) on the curing kinetics and physical properties of diglycidyl ether of bisphenol A (DGEBA) epoxy prepolymer cured with diethylenetriamine (DETA) were investigated with DSC, SEM, DMA, and tensile testing. The results revealed that FR could be a catalyst or a diluent depending on the FR content. In addition, the blending systems were partially miscible. The tensile strength and modulus of blends decreased with increasing amounts of FR, but the elongation increased with increasing FR.