The purpose of this study is to investigate the types of preposition errors that appear English diaries of Korean middle school students, and to understand the process of preposition acquisition through the analysis of preposition errors. In order to analyze proposition errors, I collected English diaries that the students wrote for a year. Their diaries were analyzed in terms of preposition errors using four criteria: types of preposition error, types of preposition, learning period, and learner level. The summary of the research results is as follows. First, as a result of research on what kind of preposition errors make in the diaries, omission errors were the most common, followed by misuse errors and addition errors. Second, it was found that most of the learners used simple prepositions and rarely used complex prepositions. Third, when comparing the early, middle, and late periods, it was observed that the students made less preposition errors over time. Fourth, the frequency of using prepositions in the top group was higher than those in the other two groups. It was also observed that the lower the English level, the higher the preposition error rate. The rate of prepositions use in the top group was much higher, but the increase rate of the prepositions in the low group was much higher in the later stages. In conclusion, there should be more activities to write down their thoughts in English writing and grammar training at the right time to contribute to increasing the accuracy of the use of prepositions.