An appropriate approach for integrating UML and B specification techniques allows us to map UML specifications into B specifications. Therefore, we can formally analyze an UML specification via the corresponding B formal specification. This point is significant because B support tools are available. We can also use UML specifications as a tool for building B specifications. Thus, an approach for a practical and rigorous software development, which is based on object and B from the requirements elicitation to the executable code, is proposed. In this paper, we address the problem of modeling UML state-chart diagrams in B, which has not been, so far, completely treated. We distinguish between event-based and activity-based parts of state-chart diagrams. We propose creating, for each part, a B specification. Because activities relate to class operations, we can use our previous work on modeling class operation for modeling the activity-based part. Hence, we consider here only the event-based part. A new approach for modeling events is proposed. The asynchronous communication amongst state-chart diagrams is also considered.