Abstract: In this paper, the main cause and the mechanism of weld porosity of Monel electrodes with slag system such as TiO₂-CaF₂ titanium type,CaF₂-CaO-TiO₂ oxide-free type and CaO-CaF₂-SiO₂ low hydrogen type were studied. Experiments showed that intensified deoxidation, denitrification and dehydrogenation in the electrode coating are the important technical measures to eliminate porosity.In hand welding of Monel metal, since nitrogen and hydrogen are the main factors causing porosity in welds, the addition of stronger denitrifiers like Ti, Al etc. into the electrode coating is definitely a necessity. The electrodes should be thoroughly dried and well baked. The physical property of the melting slag plays an important and direct role in the shape and quantity of the surface porosity. The proper selection of welding procedure and welding parameters may effectively suppress the porosity in Monel welds. Lower arc voltage, fairly low welding speed and preheating for heavy workpiece are recommended.