Abstract: The research object of this paper is the brazed CVD diamond thick film and cemented carbide using a Ag-Cu-Ti active filler alloy. The morphology and microstructure of the joint were investigated by scanning electron microscope and electronic probe micro-analysis. The new compound formed in the interface and the influence of brazing parameter on it was analyzed. The microstructure physical model of the joint was suggested.Results show that a gray interfacial reaction layer TiC is formed between filler metal and diamond under a certain brazing condition. TiCu reaction layer near TiC exists in the interface. Fracture often occurs in the reaction layer TiC, sometimes, it occurs in the TiCu layer also. Heating temperature, holding time and the composition of Ti in the active filler alloys effect the microstructure physical model of the joint between CVD diamond thick film and cemented carbide.