Abstract: The characteratics of grain boundary pcnclration in 1Cr18Ni9Ti stainless steel brazed with copper base filler metals are explained in this paper. The form and mechanism of the grain boundary penetration of copper base filler metal, being related to their wetting-spreading property are different in nature from those of boric filler metals. The said wetting-spreading of copper base filler metals is of a three-dimensional space model as confirmed by a study of Cu and CuMnCo filler metals on the surface of stainless steel. A copper-rich liquid Cu-Mn which wets and spreads along the grain boundary groove on the two-dimension surface of a base metal penetrates simultaneously the internal grain boundary as well. In general, the grain boundary penetration of copper base filler metals will not cause brittle phase and will not weaken the joint. However,it is possible that the grain boundary penetration may result in grain boundary mbritlment if stress exists during brazing.