This paper presents some initial developments in modeling design constraints of mechanical systems using predicate logic. This design constraint modeling method is to manage design changes in a concurrent engineering environment that has CAE and CAM applications. The design constraints are classified according to an object hierarchy of a mechanical system information model which has five levels object including: a mechanical system level, assembly level, and part level. The constraint relationships are defined and formally expressed using predicate logic. Feature fitting and against relationships between parts in the assembly level and relationships between features of a part in the part level have been illustrated in the paper.