The aim of this study is to suggest the method how to measure a hierarchy of difficulty in foreign student’s learning of Korean vocabulary. From the viewpoint of the traditional error analysis we assume that learner’s errors reflect the difficulty in learning language. The linguistic reasons for errors that foreign learners make in acquiring Korean vocabulary are divided into these three - interlingual transfer, semantic domains as the intralingual transfer and semantic relations as the intralingual transfer. two tests, vocabulary translation test and cloze test of 80 questions were administered to the advanced learners. A hierarchy of difficulty for each type is as follows. As for interlingual transfer, one-to-one correspondence and coalescence were the easiest types to learn, reinterpretation and split were next and the easiest types to learn, reinterpretation and split were next and the new category was the most difficult. As for the semantic relations, inclusion and complementation were the easiest learn, contiguity was next and overlapping was the difficult type. As for the semantic domains, entities and events were easier to learn than relationals and abstracts. The difficulty level for each vocabulary is calculated as the sum of the below difficulty indices. The easiest types to learn such as one-to-one correspondence, coalescence, inclusion, complementation, entities and events have the difficulty index 1. The second easiest types to learn such as reinterpretation, spilt, contiguity, relationals and abstracts have the difficulty index. 2. The most difficult types to learn such as the new category and overlapping have the difficulty index. 3. This method of measurement which is based on the complex criteria overcomes the limitation of the existing method adopting single criterion so that the former predicts learners’ difficulty more exactly than the latter. That is, the higher the difficulty level for the given vocabulary is, the more frequently learners make errors in using it. This study on error types, a hierarchy of difficulty and a method of measureme.