Test driven development uses unit tests for driving the design of the code. Mock object is an object that imitates the behavior of an object with which class under test has an association to assist the unit testing. Although many tools for mock object are used in practice, there has been few research on defining the mock object model. For the comparisons of the testing capability of the tools, it is important to define the models. In this paper we represent the models of the existing mock object tools and indicate the limitations of them. In addition we propose a new model to overcome the limitations of the existing tools. With this model, more general behavior of the class under test related to the mock object can be expressed and tested.