Students' Arabic thesis writing comprises several grammatical errors of different types. This study aimed to find out: (1) errors in writing Arabic phrases on students of the Arabic Language Education Master's Program at universities of Indonesia, and (2) factors that caused these writing errors. This research sued a qualitative research design, with the library research approach and Error Analysis techniques. The primary data comprised errors in the application of Arabic phrases from 110 student theses at three universities from 2019 to 2021. The results of this study included: (1) students' Arabic theses writing errors can be classified in four phrases, namely: murakkab ismiy (nominal phrase) of 24%, murakkab fi'liy (verbal phrase) of 28%, tarkib i?afiy (Combination of Nouns) of 26%, tarkib wa?fiy (adjective phrase) of 22%; (2) two factors caused these errors, namely: (a) language input errors, inaccuracy, and translation from an electronic dictionary without correction; (b) error caused by not heeding grammar rules, not practicing nahwu correctly, and not studying seriously the Arabic grammar. The study recommends that students should revalidate their writings and avoid copy-paste writing techniques, and lecturers should check students' writing according to grammar, especially phrases by reviewing seriously.