The heat input distribution is the important factor for penetration shape and/or occurence of weld imperfection.  When the cusp type magnetic field which can be generated with use of four magnets is applied to TIG arc, the cross section of its arc plasma changes from circular to elliptical shape.  The arc plasma is depressed by the electromagnetic force produced from the interaction between the electric current and the superposed magnetic field in the arc plasma.  In this paper described 3D numerical model of TIG arc plasma and the temperature and velocity field of magnetized arc calculated.  The calculation shows that the cusp-magnetized TIG arc gives elliptical shape of heat input on the anode surface and its arc pressure is lower than the conventional TIG arc.  The experiment shows that the cusp type magnetic field increases a ratio of depth to width in weld penetration and limitation of welding speed.