Keywords: Interference, Grammatical Interference

Grammatical interference is the fundamental problem which usually inhibit Indonesian students in acquiring English. It is caused by different grammatical structures between indonesian language and English. This factor may leads to the translation meaning and function of Indonesian language which is not necessary according to English and resulting errors. In this case, the writer chose to identify the grammatical interference among the first-year students of the English Department as an effort to prevent the problem of interferences from the beginning level of learning.
This research uses descriptive research design. The aim of this research is to identify the grammatical interferences of Indonesian language in the first-year students of the English Department writing and also to determine the types of errors that can be categorized as grammatical interference of Indonesian language. There are 14 students who were involved in this research. To measure the data the researcher uses instrument in the form of composition tests.
According to the data analysis the highest number of grammatical interferences were found in the verb inflections category (167 interference errors, 39.57 %), followed by the category of preposition (84 errors, 19.91 %), article (49 interference errors, 11.61 %), pronoun (47 interference errors, 11.14 %), plurality (43 interference errors, 10.19 %), copula ‘be’ (18 interference errors, 4.27 %), word order (11 interference errors, 2.61 %), adjective (3 interference errors, 0.75 %). None of the participants made grammatical interference in passive voice category. From 8 types of grammatical interference, the most frequent types of errors occured in the form of omission and misformation.
The research result showed that most of the students relied too much on Indonesian language. In this case, it requires the students to learn more about the differences between Indonesian language and English grammatical structures. Therefore, it is suggested for the teacher to put more attention on problematic areas where the grammatical interferences mostly happened and introduce the students about the differences that exist in Indonesian and English. Finally, the teacher is able improve the quality of teaching and learning in the classroom and overcome the problem of errors as a result of Indonesian language interference.