The laser ablation technique is one option for micro-machining and patterning of diamond film. A UV YAG laser with higher energy density can remove or destroy the diamond film more efficiently than the excimer laser. That is, the UV YAG laser not only provides faster etching rate on the diamond film, but also requires less processing and maintenance cost. In the current study, synthetic diamond films with grain size of 30 μm were deposited on silicon substrate by microwave plasma enhanced chemical vapor deposition (MPCVD) in the CH4/H2 mixture atmosphere. A pulsed UV YAG laser (λ = 355 nm, 10 kHz) was employed to machine and remove the diamond film. The diamond film surface was analyzed by SEM and Raman spectroscopy after the laser machining. The beam size of YAG laser was adjusted to between 0.1 mm and 1.5 mm by the trepan mechanism to approach the following defined scanning width. In order to shape a 4-inch diamond wafer into a microstructure, the scanning width of the UV YAG laser was defined to 0.1 mm, 0.75 mm and to 1.5 mm in several loops. The results show that the laser-polishing effect can be applied to the pretreatment of mechanical polishing of diamond wafer in the condition of 0.75 mm scanning width in 3 loops. From Raman spectrum, it could prove the mechanism of carbon burning reaction during the laser processing and the residual carbon existing in the laser-patterned area. The surface of diamond film is strongly affected by the laser processing and a better result from the parameter of 0.75 mm scanning width in 3 loops is shown in the current study.