ZnSe crystal has been widely used in infrared imaging and laser systems. However, as a typical soft-brittle material, the material removal mechanism in ultra-precision diamond turning process has not been clarified, it is still challenging to obtain nano-smoothed surface. In the study, the effect of tool ranke angle on ductile-brittle transition depth of ZnSe crystal have been investigated through novel plunge-cutting tests. The ductile regime machining model was revealed by comparing the maximum undeformed chip thickness and ductile-brittle transition depth. With the aid of FESEM, white light interferometer, and Raman spectrometer, the effect of feed rate on surface roughness, surface quality, phase transition and subsurface damage were systematically investigated. The surface defects formation mechanism was proposed. Furthermore, the material removal mechanism of ZnSe crystal in ultra-precision diamond turning process have been revealed.