Abstract: Adhesion strength of diamond coating on WC-Co cemented carbide substrate is an important factor which effects diamond coated tools performance and life. Diamond films were deposited by microwave plasma chemical vapor deposition method on cemented carbide substrates etched by HNO₃ solution. By analyzing the morphology and composition at the interface between diamond film and the substrate, the adhesion mechanisms were understood as follows: the mechanical interlocking has important contribution to the adhesion strength; the thermal stress and the weak interphases at the interface are the major factors which resulting in diamond film peeling off spontaneously.