Shearing process diagrams (the relations of shearing stress and vertical dilating strain to horizontal shearing strain) for eleven varieties of powders have been obtained to analyze the shearing-failure behaviors of the powder beds by a direct shear tester with constant vertical load. A macroscopic model has been presented to estimate the average critical void fraction and thickness of shearing zone under flowing condition in the circular shearing cell of the tester. It has been shown that the average critical void fraction and thickness are characteristic of the powders tested under identical experimental conditions.