In the program development process, it is ofren necessary for programmers to know the differences between two programs, or two different versions of a program. Since programs have structures such as iteration statement and selection statement, applying text-based tools such as UNIX diff to identify the differences may produce unsatisfactory results. In this paper, we exploit a tree as the internal representation of a program, obtain the mapping between two trees and display the program differences visually based on the mapping and pretty-printing technique so that the structural differences can be identified immediately.