This study primarily aims at identifying and classifying the most frequently occurring error types in Korean university students’ written work. This study will also try to find some ways to prevent those errors by discussing the issue of error treatment. A total of one hundred and fifteen Korean university students participated for two consecutive studies. The writing samples were collected and errors were extracted from native speaker teachers’ error correction of student writing. After the errors were identified, they were analyzed based on a modified linguistic category taxonomy (Dulay, Burt, & Krashen, 1982). In this study, the most common errors were ‘Noun Phrase’ errors, followed by ‘Verb Phrase’ errors. This possibly suggests the existence of interlanguage and implies that the differences in the two language systems or the absence of certain features in L1 (Korean) caused difficulty in their application in L2 (English) in using the correct forms. Pedagogical implications based on the findings are then discussed.