Abstract: This paper describes preliminarily the principle of the magnetic arc blow taking place in welding 9%Ni steel.The vector sum of the Lorentz force produced by the added magnetic field and the pinch effect force produced by the welding current itself determines the intensity of magnetic arc blow.If the magnetic remanence intensity of the base metal is over 60 Oe,the welding of 9%Ni steel would cause such weld defects as lack of fusion,incomplete penetration and slag inclusion.In order to ensure the qaulity of weld technological measures should be taken to reduce the action of magnetic remanence and hence to eliminate the weld defects caused by the magnetic arc blow.