In order to be a skilled performer in writing, EFL learners need to have sufficient knowledge about grammar. Therefore, grammar has an essential role in the process of mastering a language. As adjective clause appears frequently in the English textbook and reading section in National Examination, and the error in the usage of adjective clauses exists frequently in students’ writings, these phenomena draw the writer’s attention to conduct a research about these frequent errors. This research was aimed at describing the grammatical errors made by grade XI students of SMA Negeri 4 Banda Aceh when using adjective clause in writing and investigating the sources of students’ errors. The design of this study was a descriptive qualitative study. To obtain the data, the writer used document collection. The result of document collection in this study shows that the highest percentage of error type faced by students was in Concord Error and the least was Sentence Fragment, and the majority of the error source was caused by the Ignorance of Rule Restriction and the least was caused by Overgeneralization. Therefore, this study will be a comprehensive source for the teachers in organizing their point of views toward students’ errors.