Abstract: Many real systems may be high-order,time-varying and/or non-linear to the extent that conventional modeling and analysis techniques can no longer be easily applied.The use of qualitative reasoning can avoid complex mathematical operations,and can be employed to overcome some of the difficulties.Qualitative reasoning is attractive because it is involved in the description of the general principle of physical systems,which means that one particular qualitative model can be used to describe the operating conditions in a wide range.Most applications of qualitative reasoning in the engineering have so far been involved in the process monitoring and diagnosis.The paper illustrates how to apply the qualitative reasoning technology to a liquid level control system.