BiMnO3 has attracted plenty of attention due to the coexistence of ferromagnetism and ferroelectricity properties simultaneously. However, the composition evolution of BiMnO3 nanoparticles with heating treatment is not clear. Here, the co-precipitation method was adopted to synthesize BiMnO3 nanoparticles at moderate temperature to investigate the heating treatment induced surface composition evolution. The as-prepared nanoparticles were heated at different temperatures from 300 ℃ to 600 ℃ in the air for 20 min. The influence of heating temperature on crystalline structure and composition was systematically studied using transmission electron microscope (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS), respectively. The TEM images demonstrate that the diameter of BiMnO3 nanoparticles increases gradually as increasing heating temperature. The XRD results indicate that the structure of BiMnO3 nanoparticles changes from monoclinic to orthorhombic Pnma symmetry phase when the heating temperature is above 500 ℃. The XPS result suggests that the oxygen defects decrease as increasing heating temperature. Besides that, both XRD and XPS results affirm that BiMnO3 nanoparticles begin to decompose, presenting the unstable property of BiMnO3 nanoparticles in the air at a temperature above 600 ℃. The results are valuable in understanding the evolution of the composition of BiMnO3 nanoparticles with heating treatment.