Abstract: The removal of oxide layer and wetting mechanism of brazing stainless steel in vacuum is not clear so far.In this paper,Auger electron spectroscopy,mass spectrometer,vacuum thermo-balance,X-ray diffractometer,scanning electron microscope and microprobe were used for detailed investigation of these processes.Results showed that the carbon contained in stainless steel is the essential factor for the reduction of surface oxide layer in vacuum brazing.For stainless steel 1Cr18Ni9Nb,a slight amount of carbon is enough to get the reductive reaction started at about 900℃ under 2×10^-5 mbar vacuum.In vacuum brazing above 1000℃,the evaporation of oxide plays also a certain but non-essential role.With commercial brazing alloys,a brazing temperature above 900℃ is recommended.