This paper tried to investigate Korean learner's acquisition of Korean adnominal endings focusing on proficiency and learners’ native languages. For this purpose I investigated frequencies of adnominal endings in Korean learners' corpus. And I analysed Korean learners' errors when they used adnominal endings. Based on the result of analysing, Japanese Korean learners use the adnominal endings more correctly than Chinese and European Korean learners. And Korean learners have many errors using adnominal endings in intermediate and high levels as well as basic levels. What is more, advanced learners have the tense and aspect errors using adnominal endings. European Korean learners have more sentence structure errors than others. The reason why they have difficulties using adnominal endings is that the way of organizing of Korean relative clause is different from European’s.