Ultrafine grain composite diamond films are deposited on the WC-Co-based drills using a bias-enhanced hot filament CVD system. During the deposition, the spiral hot filaments and novel two-step pretreatment method for complicated shape tools are adopted. The composite structure is achieved by regulating the concentration of Ar in a mixture of acetone and a hydrogen gases at different stages. The surface morphologies and composition of the first fabricated microcrystalline diamond film and the ultrafine grain diamond film grown later are observed with different techniques. The composite films have the advantages of good adhesive strength and low surface roughness. The cutting performance of the composite diamond-coated drills is investigated by drilling SiC particles reinforced aluminum matrix composites. The drilling life is improved greatly compared with those of uncoated drills and microcrystalline diamond coated drills.