Abstract: The vacuum brazing was performed between diamond and 45 steel with Cu-Sn-Ti alloy brazing filler, and the interfacial microstructure between brazing filler and diamond was analyzed. The results show that the brazing filler and diamond had good wetting ability, the non-continuous TiC layer with no-regular shape was found on diamond surface, and the dendritic Sn-Ti compound was found on TiC layer, which played a role of transition and bridge. The diamond and 45 steel had a good metallurgical bonding.