ABSTRACT Rafiqatul Husna. 2016. A Corpus Analysis on the Grammatical Errors Committed by Accounting Department Students in Essay Writings. A descriptive Study at Aceh Polytechnic. Thesis Graduate Program in English Language Education of Syiah Kuala University. Advisors: (1)Prof. Dr. Usman Kasim, M. Ed, (2) Abdul Manan, M.Sc., MA., Ph.D. Key Words: Corpus, Analysis, Grammatical Errors, Essay Writing. This research is about an analysis on the students’ grammatical errors in essay writing. The objective of this study is to find out the types of errors, the grammatical errors, and the sources of errors which occur in the students’ essay writing. The corpus of this study was collected from the sixth semester students of Accounting Department at Aceh Polytechnic. The instruments used in this study were writing test and documentation of students’ writing. Furthermore, AntConc software was also used to help the researcher find the location of several certain words and look at the way of students in using the words. In analyzing the types of errors, this study only focuses on omission, addition, and misformation errors. Among these three errors, misformation errors were obtained as the dominant occurance in the essay writing of Accounting Department at Aceh Polytechnic, accounting for 44 (55 %) errors. Besides, for the grammatical errors, this research focuses on verb tense errors, subject-verb agreement errors, and preposition (at, in, & on) errors. This research indicates that verb tense errors are the most frequent grammatical errors identified in the students’ writing. It constitutes 33 errors or 43 % of the total number of errors. The errors committed are resulted from three causes. They are context of learning, interlingual transfer, and intralingual transfer.