Plagiarism has become one form of cheating that occurs in the academic system. Some students do plagiarism by their intelligent ways, for example by changing some grammatical structures. The problem of plagiarism must be vague for both students and teachers. This study focused on knowing the students' perception of plagiarism when doing their thesis. The data collection conducted to 20 students of English department at Universitas Islam Malang. The study done through the questionnaire using Google Forms to students. The researcher conducts the research in the Department of English Language Education of Universitas Islam Malang. With the research questions (1)what are students’ perceptions of plagiarism in completing the thesis? (2)why did English Department students at Universitas Islam Malang do plagiarism? Perception is a method for understanding or decoding knowledge acquired through the framework of human meaning. In writing with sources, plagiarism defined as the act of admission. Recognize their source — an act of deceit, cheating, cheating. The thesis is a scientific work written by undergraduate students on the subject of a specific topic or field based on literature review results, outcomes of field experiments, or outcomes of development or testing. Writing a thesis is also part of the program's science discipline. The graduation rate or the completeness of the undergraduate program measured by the quality of the results of the study that has been completed. This research is about qualitative analysis. Qualitative research is an approach to the comprehension and universal sense of individuals or groups on social issues. This study is a tool for providing multidimensional data required to gain an understanding of students' perception of plagiarism in the completion of the thesis. This research conducted through a questionnaire to collect the data needed. This questionnaire was important to explore their understanding of plagiarism. It took 5-10 minutes for each participant to answer the question. The questionnaire results were descriptive qualitatively analyzed in this study. The researcher needs to check the data multiple times from Google Forms to get the correct information from the respondents. Then the researcher writes the data to the book and reads the data. Some people think plagiarism is not good, and some think it is positive because plagiarism helps students finish their thesis. Some say plagiarism copies and pastes other works unless you put any resources, paste, or copy words without paraphrasing them, and take creations or statements without putting their source without putting a source. Some students perceive plagiarism as not a big deal and that was the reason why there was a problem of plagiarism. English Department students at Universitas Islam Malang did plagiarism. The students said they plagiarized because they had no idea how to paraphrase or summarize easy statements. They also did it because they were lazy and had limited time to complete the assignment. Plagiarism may also be positive or negative as plagiarism allows them to accomplish their tasks. English Department students at Universitas Islam Malang do plagiarism. Reasons include lack of motivation, laziness, and less honesty. The future researchers must consider gathering the data include questionnaires and detailed sample interviews to collect more knowledge. In order to obtain accurate data, the potential researcher must do an oral interview. Students should study more about the rules of citation. Lecturers should check the plagiarism of students' assignment or task using plagiarism detector.