First Language (L1) interference will inevitably occur on students in translating sentences when they have limited linguistic knowledge of target language. This study aimed at analyzing the first language interference in students when translating Indonesian sentences into English focusing on grammatical structure, lexicon, and semantic aspects. It was conducted to the third grade students of Islamic junior high school (MTsN) Kuta Baro Aceh Besar. Class IX-2 and IX-4 were chosen as the sample of this study with total 39 students. A descriptive qualitative was used as the method in this study and the data was obtained by giving some writing tests. The students were asked to translate twenty Indonesian sentences into English. The results showed that all of students of both classes were interfered by their L1, Indonesian, when translating the sentences provided by the researcher. It was indicated by their using Indonesian pattern and translating the sentences literally word for word which led them in producing errors in term of grammatical structure, lexicon choice, and semantic. Thus, the problem of learning EFL in MTsN Kuta Baro could be largely due to L1 interference. Therefore, remedial measures should be taken to assist the students in these problems.