This project entitled “FIRST LANGUAGE INTERFERENCES INTO ENGLISH WRITING SKILL OF THE XIIth GRADE STUDENTS OF SMA NEGERI 1 KUPANG IN ACADEMIC YEAR 2017/2018.” This research is aimed at; 1) figuring aspects of first language (L1) interferences; and 2) the affecting factors of L1 interference into English writing skill. By applying descriptive qualitative approaches, there were some strategies using in both compiling and analyzing data. In collecting data, triangulation was applied such as interviewing teacher and students, classroom observation, and students’ writing assessment. The data were obtained by interviewing students and teacher and assessing writing skill of students of SMA N 1 Kupang in academic year 2017/2018 in the third grade of language program. Typology was applied in analyzing data using contrastive analysis. The result of this investigation proved that there are four levels of L1 interference found in students’ English writings. They are phonology, morphology, syntax, and semantics. There are also four affecting factors that cause the occurrence of L1 interferences. They are mental process happening in students’ brain, students’ L1, Teaching and Learning Process (TLP) model in classroom, and under-frequency of English use. Mental process in Students’ brain and students’ L1 are novelty dimensions of this research.