ABSTRACT Maya Irmalia, 2015.An Analysis of Indonesian Grammatical Interference on Senior High School Students’ English Writing Task (A Case Study at Senior High Schools inSigli).Thesis. Darussalam, Banda Aceh. Advisors : (1) Prof. Dr. Bahrein T. Sugihen, M.A (Emeritus) (2) Dr. Sofyan A. Gani, M.A Keyword : interference, grammar, writing, This research deals with Indonesian grammatical interference on students’ English writing. This research was designed to investigate: (1) the most common Indonesian grammatical interference found on the English writing task of the students of senior high schools inSigli, and (2) the factors that caused the students of senior high schools in Siglito be interfered by Indonesian grammar. The subjects of this research were 86first graders and three English teachers of the three senior high schools: 29 students of SMAN UnggulSigli, 27 students of SMAN 1 Sigli, and 30 students of MAN 1 Sigli andone English teacher of each school. The data of the research were collected by using document analysis, questionnaire, and interview. The document analysis was in the form of the students’ English writing papers about the description of their friends related to the appearances or personalities. The result of the document analysis shows that the incorrect verb tense, including subject verb agreement, is the most common Indonesian grammatical interference made by the students in their English writing, which is around 55%. Based on the result of the questionnaire and interview, it was found that the L1 interference made by the students wasdue to the students having little practice to improve their English. Besides, the teachers did not provide the students with good English environment which can make them more familiar with English.