The cutting mode transition from ductile to brittle is related to the depth of cut for the machining of brittle materials. The critical depth of cut for ductile to brittle transition (DBT) during single-point diamond turning of single-crystal calcium fluoride was examined, and the effect of cutting direction, cutting speed, and tool rake angles were investigated. Results show that the cutting speed had a slight effect on the critical depth of cut for DBT, while negative rake angle tools yielded large critical depth of cut for DBT. The influence of cutting direction (crystallographic orientation) on the critical depth of cut for DBT was associated to fracture toughness (KC) of the materials. Higher KC values induced larger critical depth for DBT. Furthermore, periodic variations of KC values as a function of the crystallographic orientation correlated well with changes in critical depth ranging between 100 and 600 nm. This resulted in the successive emergence of brittle and ductile cutting regions when a nominal depth of cut of 0.5 μm was used, while it led to the formation of a smooth and homogenous surface with Ra of 2.838 nm at a nominal depth of cut of 0.1 μm.