The ductile–brittle transition was investigated based on material crystallography and stress intensity factor on the four distinctive planes of sapphire. Cutting forces were analyzed to explain the anisotropic transition characteristics. The projected stress on the objective plane at the transition point was estimated from the critical stress intensity factor of the corresponding crystal plane. Per cutting direction, the effect of crystal planes on the machinability was discussed, and the estimated stress was compared with measured values. Experimental results showed that the anisotropic machinability can be predicted in terms of crystal orientation and the critical stress intensity factor.