Generally, web services are provided with different QoS values, so they can be selected dynamically in service composition process. However, the conventional context free composition QoS model does not consider the changeability of QoS values and the context sensitive constraints during composition process. In this paper, we propose a rule based context sensitive QoS model to support the changeability of QoS values and the context sensitive constraints. By considering context in the QoS model, web service composition can be used widely and flexibly in the real world business. Categories and Subject Descriptors H.3.5 [Information storage and retrieval]: On-line Information Services ?Web-based services; H.3.3 [Information storage and retrieval]: Information Search and Retrieval ? Selection process. General Terms: Algorithms, Economics, Measurement.