Although program fault are widely studied, there are many aspect of faults that we still do not understand, particularly about OO software. In addition to the simple fact that one important goal during testing is to cause failure and thereby detect fault, a full under tanding of the characteristic of faults is crucial to everal research area. The power that inheritance and polymorphism brings to the expressiveness of programming language also brings a number of new anomalies and fault type. This paper present a model for the appearance and realization of OO fault and defines and discusses specifc categories of inheritance and polymorphic fault. The model and categorie can be used to support empirical investigations of object-oriented testing technique, to inspire further research into object-oriented testing and analysis, and to help improve design and development of object-oriented software.