Abstract: In a gravity field, the changes of fluid density cause natural convection, while in a magnetic field, the apparent density of magnetic fluid changes markedly with the change of the intensity of external magnetic field.Therefore,magnetic field can influence the natural convection of the magnetic fluid.Through static calculation and analysis,a synthesized magnetic field can be established resulting from the coaxial superposition of a uniform magnetic field and a uniform gradient magnetic field.The change of the density of magnetic fluid caused by the synthesized magnetic field is equal everywhere in the fluid, and natural convection heat transfer is also equal everywhere.Thus the apparent density and the coefficient of natural convection heat transfer can be accurately measured and a correlation equation can be established between magnetic field intensity,apparent density and the Grashof criterion of the magnetic fluid.Accordingly, the increase of magnetic field intensity causes an increase of apparent density of magnetic fluid, which is magnetic fluid in a state of overweight.That is the mechanism of natural convection heat transfer of magnetic fluid enhanced by magnetic field.