Abstract
This study aims to know UIN Antasari students’ common mistakes in writing Arabic annexation system (idhafat). The sample is 90 students who join in the subject of tarkib tawabi. It takes qualitative approach with error analysis method. The data are collected from the documentation of students’ syntax workbook which is then analyzed by syntactic error analysis. The result of the study shows that the common mistakes in writing Arabic annexation vary from the mistake in mudhaf, mudhaf ilaih, multiple idhafat, and that in meaning of idhafat. These errors are caused by inadequate duration of learning Arabic in the secondary school level, the teacher that focuses more on explaining syntax theory, and the school’s orientation in raising students’ speaking skill.