This paper will focus on how first language interference occurs when Korean students learn Japanese writing. Since there are similarities in grammar between Korean and Japanese, it is said that Japanese language is easy to learn for Koreans. However, that is only true for the beginner level. As the level of Japanese competency goes up, Korean students have considerable difficulty learning some Japanese language features, which are seemingly similar but have subtle difference from Korean language such as use of postpositional particles and active vs. passive voice. Such first language interference is well shown in the errors that Korean students made in the Japanese writing test for Japanese University Admission for International Students. A high frequency of errors occurred in postpositional particles, active and passive voice, syllable order, and Japanese spelling of foreign words including IT terminology. Those errors are the examples of negative first language transfer. Thus, it is important to teach strategies on how to avoid first language interference in Japanese writing class for international students.