Recently, as the importance of IT management is increasing, many companies have adopted ITSM(IT Service Management) as a solution to various IT management issues. In addition, some advanced companies are using 6 Sigma technique in their implementation of ITSM. The main purpose of this study is to analyze critical success factors of 6 Sigma in applying to the implementation of ITSM. For a case company, which is known to be successful in applying 6 sigma in its implementation of ITSM, six critical success factors were analyzed. The factors include leadership of CEO, capability and level of 6 Sigma belts, management of accurate data, building organizational systems, training and education, and evaluation and incentive. The results of the study are expected to be used as an useful information for companies to apply 6 Sigma in the implementation of ITSM.