Using Ni-Fe alloy as the filler metal, the brazing joining is realized successfully between diamond and the steel substrate under the conditions of different temperature and time of brazing. The repeatability of experiment results is very good. With the help of the microscope, the diamond distributions on the filler metal are observed, and the surface and section of the brazing sample are observed carefully using SEM. EDS gives the existent elements and the content distributions of element in small area after brazing. XRD gives the existent phases of substance in filler metal and diamond. The experiments show that: the element of Fe in Ni-Fe alloy diffuses to the diamond surface and reacts with C in diamond so as to form a thin layer of carbide Fe_5C_2 at the interface between diamond and the filler metal. Because of the carbide, the brazing joining is realized between diamond and the steel substrate. From the wetting theory of liquid to solid and the theory of dissolving and diffusing, we can conclude further that: diamond dissolves into the filler metal and the filler metal diffuses to diamond, which forms the carbide. The carbide ameliorates the wetting of the filler metal to diamond, promotes the brazing ability of the filler metal to diamond.