For effective use of parallelizing compilers, an interactive environment which allows users to find more parallelism is needed. As the first step towards building such an environment, we have developed a program visualization system named NaraView. In this paper, we describe two visualization methods in NaraView. One is Program Structure View which illustrates the hierarchical loop structure of a given program and suggests which parts of the program can be parallelized. Another is the Data Dependence View which visualizes each data dependence on every variable or array element which is accessed in a specific loop. By using these views, users can easily understand which part of the program can be parallelized further. We also show several examples to demonstrate the efficiency of these methods.