In order to explore the ablation characteristics and the material removal mechanism of diamond in picosecond laser processing， an experimental study of picosecond laser processing microgrooves on CVD single crystal diamond was carried out. The field emission scanning electron microscopy was used to observe the external and internal morphologies of the diamond microgrooves. The experimental results show that micro-chippings and micro-cracks were emerged at the edge of the diamond microgrooves， and nano-rippes with periods of approximately 255 nm and 495 nm were formed at the sidewall and bottom of the microgrooves. The ablation threshold， ablation rate and material removal rate of diamond were obtained by measuring the width， depth and volume of the diamond microgrooves. Raman analysis shows that graphite was generated at the bottom of the microgroove. This indicates that the ablation process of picosecond laser processing diamond proceeds via a surface graphitization. the graphite peak red shifts as the laser energy increases. Theoretical results show that the thickness of the graphite layer at the bottom of the diamond is about 88.7 nm. The temperature field of the picosecond laser ablation of diamond was simulated. The simulation results show that the laser energy is mainly distributed on the surface of the diamond， while the laser energy diffuses into the diamond through heat conduction is very small. Therefore， the heat-affected zone of the picosecond laser processing diamond is extremely small， resulting in the thickness of the graphite layer is less than 100 nm.