Under-cut which occurs occasionally in the metallic arc welding is unaccountable phenomenon. We attend to trace the Under-cut to its origin and to describe necessary conditions to promote this occurrence. As the result of the experiment mechanism of under-cut was illustrated. We can say that it is the natural consequence of the arc welding and conditions to promete the occurrence of it 'are as follow : higher current dencity, higher welding speed, Sigher temperature of mother metal, lower melting point of mother metal, rust of the metal used, to use characteristic coated electrode such as coating material of higher arc voltage, bad fluidity of metal, unsuitable fluidity and freezing point of slag. And finally, improper manipulation of welder.