CVD diamond thick film was brazed to cemmented carbide using a Ag-Cu-Ti active filler metal in vacuum furnace. The interfacial microstructure and characterization between diamond and Ag-Cu-Ti filler metal under different processing condition were studied. For analyze the interface microstructure between diamond and filler metal, the filler metal was peeled off layer by layer toward the diamond film and at the same time the interface microstructure and distribution was examined layer by layer by X-ray diffraction. The fracture surface and across surface of brazed joint were examined by SEM, EPMA, EDX. Results illustrated that Main element Ag,Cu and a small of new compound TiC,TiCu compound existed in the interface.The morphologies and distribution of new compound were found for the first time (Figs).TiC layer closed to diamond appears gray and it's thickness is variational with varying processing condition such as peak heating temperature,keeping time and the content of Ti element in the filler metal.The thickness is about 1µm(see Fig) under such processing condition: composition of filler metal is Ag-Cu-4Ti, peak heating temperature is 1190K,keeping time is 15 min.TiC grows in the surface of diamond under a special direction at the beginning. According to the Bramfitt equation about the planar disregistry between nucleant and substrate,the most suitable grow direction of TiC was obtained that is (111)TiC// (001)diamond.The microstructure model near interface in order is diamond / TiC / TiCu + (Ag-Cu). The key factor to join diamond film with substrate firmly is forming TiC in the interface by C in diamond and Ti in filler metal. But to much TiC in interface can weaken join strength of joint because it is brittle.