Software statistical testing is concerned with testing the entire software systems based on their usage models. In the context of UML-based development, it is desired that usage models can be derived from UML analysis artifacts. This paper presents a method that derives the software usage models from reasonably constrained UML artifacts. The method utilizes use case diagrams, sequence diagrams and the execution probability of each sequence diagram in its associated use case. By projecting the messages in sequence diagrams onto the objects under test, the method elicits messages and their occurrence probabilities for generating the usage model of each use case for the objects under test. Then the usage models of use cases are integrated into the system usage model. The integration procedure will utilize the execution sequential relations between use cases.