Natural convection of a magnetic fluid is different from that of Newtonian fluids because magnetic body force exists in an addition to gravity and buoyancy. In this paper, natural convection of a magnetic fluids (W-40) in annular pipes was studied by experimentally. Inside wall was kept at a constant temperature (25 $^{circ}C$), and outside wall was also held at a constant but lower temperature (20 $^{circ}C$). The magnetic fields of various magnitude were applied up. This study has resulted in the following fact that the natural convection of a magnetic fluids was controlled by the direction and intensity of the magnetic fields.