Web services and their underlying system grow over time and need to be retested whenever there is a change. This is essential for ensuring uncompromised quality. If we have modified only a small part of the system, it should be possible to reuse the existing test suite. Anyhow, for large modifications or for large systems, retesting the entire test suite will consume large amounts of time and computing resources. In this paper we propose a new method to prioritize test cases in web applications. Our test prioritization technique orders test cases in such a way that the most beneficial is executed first. Most of the existing test prioritization methods are based on the code of the system, but we propose a model-based test prioritization using activity diagram. Our technique identifies difference between original model and modified model. Using this information we plot activity paths for each test case and identify the most promising paths. The test case which covers these paths is considered as the most beneficial test cases. Our approach is effective in revealing the most promising regression test cases. We have applied our method on an online air ticket reservation system in which we could identify the most beneficial test cases from the existing ones.