Abstract: Originality is one of the most important indicators to value the quality of MA theses. The study of plagiarism has become a growing concern nowadays in the world to help people to have good quality of their MA theses. As for the research methods, this paper is conducted in a rather large scale and adopts an on-line detection tool to study the plagiarism of English majors in their MA theses. The findings of the present study show that below 30% of plagiarism covers 79.6% of the theses; 31-50% of plagiarism, 19.2% of the theses; above 50% of plagiarism, 1.2% of the theses. These findings could help MA students to improve their academic writing for reducing plagiarism.