For effective use of parallelizing compilers, an interactive environment which allows users to instruct the way of parallelization is needed. As the first step to build such an environment, we have developped a program visualization system named Nara View. The system provides two powerful methods for 3D visualization of program structure and data dependence. 3D visualization of program structure illustrates a hierarchical loop structure of given programs and suggests which parts of the program have been parallelized. 3D visualization of data dependence explains each data dependence on any variable or array element which is accessed at a specific loop. By using these methods, users can easily understand which part of the program should be more parallelized. We also show several examples to demonstrate the efficiency of these methods.