This study aimed at investigating linguistic errors committed by students majoring in Non-English Department in writing English composition. A total of 20 first year students at Economic College IBBI Medan who have taken an English subject course were involved in this study. Their compositions were analyzed for the purpose of scrutinizing linguistic errors they made. In analyzing the data, this study adopted Hubbard (1996) classification of errors including grammatical, syntactic, substance, and lexical errors. The results of data analysis discovered that students committed 352 errors in their compositions with the following frequencies: 82 errors in propositions, 32 errors in tenses, 29 errors in articles, 48 errors in subject verb agreements, 37 errors in word order, 17 errors in capitalization, 21 errors in punctuation, 28 errors in spelling, and 58 errors in word choice. Hence, it is concluded that the most difficult linguistic error that students face is prepositions, while the easiest part of linguistic error that poses difficulty for students is capitalization.