The purpose of this study is to investigate error patterns in EFL college students’ English writing as well as their change over time, based on the teacher’s feedback. In order to accomplish this purpose, two research questions were constructed; first, what are the characteristics of Korean EFL students’ writing based on the maturity of English sentence by the T-unit analysis? Second, what types of error patterns are produced in Korean EFL students’ writing? Also, how do the error patterns change based on the teacher’s feedback over time? The participants were four Korean EFL college students, and they were asked to pre-write, draft, revise and edit until they completed their final draft. The results of pre- and post-writing test were also analyzed. The major findings are as follows: 1) The mean number of T-unit among participants was 42.25 units, and the mean number of words per T-unit was 10.95 words. 2) The most frequently committed errors were found out as lexical and morphological errors. Moreover, the rate of lexical and sentence structure errors has been dropped, whereas the rate of punctuation errors has increased as the teacher’s feedback progressed over time. Pedagogical and practical suggestions are also made on the effective teaching of English writing in Korean classroom settings.