This paper (1) introduces Natural Assembly Relationships (NARs) to model assembly constraints between parts and (2) analyzes the constraint relationships that are defined using NARs for parameterized mechanical assemblies. The NARs are expressed as first order predicates using primitive features as arguments, such as planar and cylindrical surfaces. The concept of Basic Parameters Set is introduced as the design parameters in CAD systems, which, together with a set of form feature constraint relationships, maintains maximal consistency with original design meanings. Based on NARs, part parameters, and primitive features, design parameter chains can be constructed for closed loop assemblies. From these chains, constraint equations can be derived, which are the basis for change propagation and information management for mechanical system design.