The aim of this study was to examine Korean high school students` difficulties in the use of English relative pronouns through the error analysis. 160 first-grade students at a high school in Gwangju were participated in the study and they were tested using 37 items, which contained English relative pronouns. Student errors were divided into 3 parts: 1) a grammatically judgement task, 2) a gap-filling task, and 3) a task comparing restrictive with nonrestrictive function. The analysis of grammatically judgement task showed that the students had the most difficulty with the use of an appropriate relative pronoun omission. Whose was ranked as the highest error rate and who as the lowest. The degree of students` understanding about the restrictive relative pronoun was examined by analyzing the number of correct answers. It showed that the hierarchy of difficulties from the easiest to the most difficult was as follows: OS>SS>OO>SO. The study also suggested some ways to help the students acquire English relative pronouns effectively. The most important thing in teaching relative pronouns was to provide meaningful contexts in which students have enough opportunities to practice them.