In assembly operations, there are various constraint states while a moving object is in contact with a fixed object. For different constraint states, the existent regions of contacting forces, geometrical constraints, possible object displacements, which can be expressed by polyhedral convex cones respectively, are different. We consider that the sizes of these regions are important characteristic of contact stability for planning assembly operations. This paper proposes an approach that can evaluate the sizes of these regions. Based on this approach, the magnitude of form constraint, the constraint maintainability and the constraint separability are defined to evaluate the contact stability and contact condition between objects. And then, their applications to planning of assembly tasks are discussed.