Software for medical image analysis and visualization is an important tool in medical research and diagnosis. Development and usage of medical image processing toolkit have become an important research trend. It is a fundamental task to master their image rendering principles and data organization mode in order to use these toolkits. MITK (Medical Imaging Toolkit, from www.mitk.net) with the important characteristics of inheritance and virtual function uses the Object-Oriented design approach. This paper researches the data organization mode and coordinate translation of MITK and presents an algorithm based on OpenGL to rewrite the image-rendering function of MITK. Using the algorithm, the rewritten rendering function has the same rendering effect as the one in MITK. By rewriting the virtual image-rendering function and using the capability that a view window can load many data model, users can neatly render various graphics and images. Though the researches of this paper are based on the mitkImageModel class, the algorithm also is fit for 2D widgets class.