Recent results on the ultra-precision machining of plastics by a single crystal diamond tool with a highly precise rigid cutting apparatus are described in this paper. Using a newly developed ultra-precision air spindle and slide, detailed experiments were carried out for polymethyl methacrylate, polycarbonate and ally di-glycol carbonate among eight kinds of plastics, under different cutting speeds, depths of cut and feeding rates with several kinds of cutting tools. Chips from plastics cutting, cut surfaces and surface roughness were observed. It was clarified that these phenomena depend on characteristics of plastics. As a result, it was found that PMMA is the most suitable material for obtaining an optical grade finish by a single point diamond cutting.