Model-based diagnosis is a new intelligent diagnosis technique which can overcome the shortcomings of traditional diagnostic methods. Hierarchical diagnosis is an important method for reducing the complexity of model-based diagnosis. In this paper, hierarchical description of the device to be diagnosed is proposed and the relationships between different abstract levels are pointed out. The soundness of hierarchical diagnosis is also proved: a diagnosis at the abstract level has a corresponding diagnosis at the detailed level. Furthermore, the incompleteness of hierarchical diagnosis is pointed out: a diagnosis at the detailed level may not have a corresponding diagnosis at its abstract level.