To support the efficient detection of theft of Java class files, this paper presents a new method to derive birthmarks from given Java class files. The proposed method extracts from a class file a set of unique characteristics, the birthmarks, based on constant values in Beld variables, the sequence of method calls, the inheritance structure and used classes. By using the birthmarks, we can easily identify the doubtful class files (those which seem to be copies). Two experiments were conducted to evaluate the proposed method. The first experiment showed that the proposed birthmark successfully distinguished non-copied mes in practical Java applications (97.50%). In the second experiment, it was shown that the proposecl birthmark had quite a good tolerance for program optimization (97.30%).http://library.naist.jp/mylimedio/dllimedio/show.cgi?bookid=100038791&oldid=7132