A new shear angle prediction law is proposed for oblique cutting operations. Assuming a thin shear zone model, the oblique cutting mechanics are described by the five angles which show the directions of shear, resultant cutting force and chip flow. Five expressions required to solve the unknown angles are derived from the kinematics of oblique cutting and the physical law of deformation, i.e. either Maximum Shear Stress or Minimum Energy Principle. Unlike the previous solutions which require intuitive or empirical assumptions, the proposed methods use only the tool geometry and the material properties, i.e. shear yield stress and average chip-rake face friction coefficient. The predicted angles and forces agree well with the empirical and experimental results reported in the literature. Furthermore, the proposed models are experimentally verified in predicting forces for practical oblique helical end milling operations.