Abstract: From the stand points of the methodological principles of object-oriented approach and thetechnical characteristics of object-oriented programming, the paper highlights the objects.class and the cor-responding class hierarchy and class composition,which enhance the capability of the O-O system in de-scribing the complex structure of the real world. The O-O system is flexible in problem solver selection and solving strategic design as the operations areorganized by message passing. This point is extremely important to the designand implementation of intelli-gent decision support systems(IDSSs),which strongly require the functions of human-machine interactionand their cooperation in the solving processes. The paper gives an illustration example of model objects andmessage passing mechanism of a modeling case implemented by language C⁺⁺.