In order to improve the adhesion between diamond coatings and cemented tungsten carbide (WC-Co) substrates, the diamond coatings were deposited on one kind of cobalt-deficient gradient WC-Co by the microwave plasma chemical vapor deposition (MPCVD). Scanning electron microscopy, X-ray diffraction and Raman spectroscopy were used to characterize the diamond coatings. The results showed dense, well facet diamond coatings. The cobalt content at the surface of substrate was measured by electron probe microanalysis. It was found that Co did not largely move to the surface as usual with deposition time increasing compared with the conventional tungsten carbide; The cobalt content at the surface of substrate after deposition (about 1 wt %) was lower than before (3.42wt%), which improved diamond coating’s adhesion against the tungsten carbide substrate.