The metamodel techniques and model transformation techniques provide a standard way to represent and transform data, especially the software artifacts in software development. However, after a transformation is applied, the source model and the target model usually co-exist and evolve independently. How to propagate modifications across models in different formats still remains as an open problem. In this paper we propose an automatic approach to synchronizing models that are related by model transformations. Given a unidirectional transformation between metamodels, we can automatically synchronize models in the metamodels by propagating modifications across the models. We have implemented a model synchronization system supporting the Atlas Transformation Language (ATL) and have successfully tested our implementation on several ATL transformation examples in the ATL web site. Categories and Subject Descriptors D.2.2 [Software Engineering]: Design Tools and Techniques General Terms ...