This paper deals with the mechanism of surface heterogeneity due to crystallographic anisotropy effects in diamond turning of single-crystalline germanium. A microplasticity-based numerical simulation model was proposed, in which the effects of tool geometry and machining conditions can be involved. Two coefficients were introduced to compensate the Schmid factors of two different types of symmetrical slip systems. Simulation of ductile machinability was conducted on two crystallographic planes (100) and (111), and the simulation results were consistent with the experimental results. It was indicated that the simulation model can be used to predict the brittle-ductile boundary change with machining conditions and crystal orientations of germanium.