Model transformations, the core operations within model-driven development, are usually realized as special kinds of programs. They can be used to achieve diverse conversions among models, codes, and even structural documents. With the rapid progress in model-driven methodology, model transformations are being applied to more and more complicated problems in industrial projects. As a result, they become large in scale and have complex structures. To handle the development complexity of large transformation, a graphical modeling language that can serve as a user-friendly notation for analyzing and designing transformations is required. This paper proposes VisTML (the visual transformation modeling language), a visual modeling language for model transformation programs. VisTML is comprised by seven diagrams, including goal diagram, transformation declaration diagram, model type diagram, rule diagram, composite transformation diagram, testing diagram, and configuration diagram, each of which includes a set of concepts derived from concrete transformation technologies. VisTML is able to cover all the major phases when a transformation is being developed. It supports developers to describe a transformation from various viewpoints at different abstraction level. Modeling a transformation using VisTML can help developers control the complexity and facilitate their communication. Then, the tool support of VisTML named TModeler is also presented. At last, three case studies are presented to demonstrate the feasibility and the effectiveness of VisTML.