The purpose of this study is to identify critical factors for successful Six-Sigma implementation by using the Analytical Hierarchy Process (AHP) approach. Twenty-one experts, consisting of three-project champions and eighteen-black belts from five multinational companies (Singapore, Japan, and USA) locating in Thailand, were interviewed. The study has gone through four phases, including: (1) determining critical factors and performance measures in Six-Sigma implementation; (2) structuring the problem and building the AHP model; (3) collecting data from expert interviews; and (4) determining the normalized priority weights of individual factors and sub-factors. Expert Choice software was used to compute the normalized and unique priority weights. The result of data analysis determined the relative importance of individual factors and sub-factors, and in turn identified the critical factors on which organizations should put their efforts throughout the process of 6-Sigma implementation.