In this paper, we propose a new model called Behaviour Description Equation which solves the problem of inheritance anomaly found in concurrent object-oriented programming languages and provides a means for the reuse of synchronisation code. The fundamental reason why inheritance anomaly occurs is that for a concurrent object, when synchronisation code is not properly separated from the method code, the extension of code to produce a derived class may force the change of both the synchronisation code and method code in the super class. We believe that both synchronisation code and method code should be treated in a separated manner as two different inheritance systems so that the interference between inheritance and object-based concurrency is avoided.