Plagiarism is often considered as cheating, dishonesty, copying, or moral failing in writing because it is the act of stealing others' language and ideas without proper citation or paraphrasing. However, this idea is not universally shared because people from different cultural backgrounds are likely to conceptualize plagiarism differently. To fill the gap in teachers' perceptions and investigate students' actual plagiaristic behavior, this study aims to detect the extent of plagiarism in students' English summary writing by adopting plagiarism-detection software, Turnitin. The present study draws on both quantitative and qualitative data to investigate whether discrepancy exists between students' perceptions of plagiarism and their actual plagiaristic behavior, and whether student behavior changes after their awareness training on plagiarism. The results of this study show that student plagiaristic behavior has changed and the awareness of adoption of plagiarism-detection software significantly has reduced the instances of textual plagiarism. It is thus noted that students who were aware that a plagiarism detection system was in use had lower percentages of plagiarism. However, no obvious relationship is observed between plagiarism awareness and the students' actual plagiaristic behavior. The implications of these findings on pedagogical practice and future research are discussed and presented.