In this study, Fe-Ni bimetal oxides (Fe-NiOx) catalysts were used to activate O3 in a heterogeneous system. This research focuses on the degradation mechanism of dimethyl phthalate (DMP) by adding Fe-NiOx catalysts which activated the O3 decomposition to generate strong oxidative hydroxyl radicals (•OH). The experimental results showed that O3/Fe-NiOx catalyst calcinated under 500 °C achieved 47.75% higher DMP removal efficiency when compared with ozonation of DMP without the catalyst. Besides, control experimental data suggested that the adsorption of DMP was negligible (<4%) for Fe-NiOx. Thus, it was concluded that the removal of DMP was due to the catalytic ability of Fe-NiOx rather than the adsorption of DMP onto the catalysts. Moreover, the unsatisfactory performance of Fe-NiOx catalysts calcinated in the presence of N2 revealed that O2 played an important part in affecting the crystalline degree of catalysts during the synthesis process. Meanwhile, the X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR) characterization analysis indicated that calcination temperature was an indispensable factor that influenced the relative content of Ni2+ and hydroxyl groups (−OH) in Fe-NiOx catalysts. Furthermore, the relative content of Ni2+ had a greater effect on catalysts' activity because of the electron transfer between multivalent metal states.