The work aims to study the processing of single crystal diamond by ultraviolet laser and analyze the trench effect. In vacuum, a single crystal diamond was irradiated by a pulse laser with 248 nm(KrF) wavelength, 500~700 mJ pulse energy, 1~9 Hz pulse frequency and 0.5~6.3 W laser power. The generated plasma was diagnosed by optical emission spectrum (OES). The trenched diamond was characterized and analyzed by Raman spectrum and scan electronic microscope (SEM). The trench was found on the diamond surface after laser irradiation. The trenching rate was obviously subject to laser frequency till the pulse frequency arrived at a threshold value to keep the trenching rate unchanged as the pulse frequency and energy. The laser-induced plasma wasmainly composed of C3, while C1 and C2 were ones else. The influence of laser frequency and energy on the trenching rate ismainly due to the laser-induced plasma. Besides, the surface of the micro-groove of diamond issmooth without obvious cracks after laser trenching.