Various brittle–ductile transition (BDT) criteria have been developed in the literature to estimate the critical conditions for ductile microcutting of brittle materials. This study provides a unified criterion to efficiently and accurately estimate the critical condition based on the indentation model on brittle materials. The unified criterion correlates with the cutting edge radius, material properties, and a dimensionless coefficient fitted by the experimental data. It shows that the cutting edge geometry is the dominant factor and the maximum undeformed chip thickness (MUCT) can be used as the unified criterion in BDTs. Based on the proposed model, microturning and micromilling have been analyzed to determine the threshold value of the MUCT for ductile microcutting. The model has been validated by the experimental data. Based on the models and three-dimensional geometrical model of microcutting, a further analysis shows that the process conditions greatly affect the microcutting efficiency even though all the conditions may achieve the ductile-regime cutting.