The pure phase BiFeO3 nanoparticles with different sizes are prepared by an ethylenediaminetetraacetic acid complexing sol-gel process. The structure, the morphology and the magnetic properties of BiFeO3 nanoparticles are characterized by X-ray diffractometry, scanning electron microscopy, superconducting quantum interference device magnetometer and Mossbauer spectra analysis. The results show that the BiFeO3 nanoparticle possesses an weak ferromagnetism at room temperature with a strongly size-dependent property. It reflects that the weak ferromagnetism present in the BiFeO3 nanoparticle should be attributed to the grain size-confinement effect, rather than the existence of the magnetic impurity or Fe2+.