Branched polyester amide containing various amount of mix rare earth oxides (RE2O3) were synthesized by in-situ solution condensation of AB(2) monomer synthesized based on the reaction between maleic anhydride and diethanolamine (DEA). The effect of AB(2) monomer self-polycondensation on the RE2O3 surface and the properties of synthesized composites were investigated by using Fourier-transform infrared spectra (FT-IR), ultraviolet-visible absorption spectra (UV-vis), X-ray photoelectron spectroscopy, scanning electron microscopy, thermogravimetric analysis and viscosity determination. AB(2) monomer can envelop RE2O3 particles as an organic matrix (HBPEA') by self-polycondensation under the proposed condition. Ionization of surface of RE2O3 aggregates can result in the rare earth ionic (RE3+) and the resulting RE3+ might coordinate with the organic matrix. Thermal stability of branched polyesteramide can be significantly improved in presence of 5 wt% RE2O3, possibly due to the coordination reaction between RE2O3 and the active group of the organic matrix.