The heat transfer and fluid flow by natural convection of magnetite/ferrite magnetic fluids under vertical magnetic field was also investigated in a rectangular horizontal container with free surface．The bottom plate was heated uniformly．We evaluated an influence of the vertical magnetic field on the heat transfer and fluid flow．The temperature field on the free surface was visualized using infrared thermography．It was found that the magnetization characteristics affect the heat transfer and fluid flow by natural convection of magnetic fluid under the vertical magnetic field in addition to the rheological effect．