The orthogonal cutting tests were carried out on an ultra precision fly-cutting machine at various depths of cut from 3 μm down to 0.0015 μm (1.5 nm). The cutting force, thickness of chip and contact length between the chip and the rake face of tool were measured in the experiments. The effect of the depth of cut on the cutting force, the shear angle, the tool-chip contact length and the cutting heat were presented and compared with the results of conventional orthogonal cutting process. The compatibility and the limitation of application of basic orthogonal cutting model to ultra-precision cutting was investigated. The experimental data obtained by ultra-precision diamond cutting have a good agreement with the shear angle equations based on basic orthogonal cutting model under the conditions of depth of cut more than 0.2 μm but not at depths of cut smaller than 0.2 μm due to the rounded edge of tool.