A set of inequality constraint conditions of the strength parameters is derived from the convexity of the strength criterion，which is expressed in a quadric form in terms of stress components. The constraint conditions can be used to identify the availability of the strength，criterion and they are helpful to measure the basic strength parameters. A geometric explanation to the strength criterion is obtained when the strength criterion is expressed in a principl stress form，which is an ellipsoid，such as elliptic， paraboloid，revolutionary paraboloid，e1liptic cylinder or cylinder etc.