Improving adhesion and surface roughness of diamond films on WC–Co substrate is the key factor of the widespread application of diamond coated tools. A new pretreatment method has been performed for smooth Co-cemented carbide inserts in order to lower the surface roughness of diamond films under the premise of good adhesion between diamond films and substrates. The effect of the new pretreatment on the adhesion of the diamond films is investigated. Research results show that the boronization pretreatment can effectively suppress cobalt diffusion to the surface and avoid catalytic effect of Co at high temperature. This new pretreatment can avoid the surface roughening of inserts and ensure the deposition of smooth diamond films. Investigation shows that the optimum boronization compounding is a powder mixture of 70%B4C+15.5%KBF4+1.5% La2O3+13%Na2CO3. Adhesion between substrates and diamond films is evaluated by Rockwell A indentation tests and the cutting performance of the diamond-coated tools is investigated by the cutting tests. Diamond films on smooth cemented carbide inserts with cobalt boride interlayer have high adhesive strength and low surface roughness. Diamond-coated tools with boronization pretreatment have a 5-fold increase in tool life compared with untreated ones.