Four samples of Fe3O4 microcrystals with average particle size of 72, 88, 100, 100A have been investigated by Mossbauer spectroscopy, accompanied with x-ray diffraction and transmission electron microscopy. The experimental results show that the anisotropy energy constantK increases as the particle size decreases. It is found that a sudden drop ofK exists around Verwey transition temperatureT v andK remains constant in temperature well aboveT v . The Verwey temperatureT v for Fe3O4 microcrystals with different particle size is almost the same as that of bulk magnetite.