This study suggests a design methodology of structural beam of modular buildings using Six-sigma. Although Six-sigma has been implemented in the manufacturing and other service industries, it is still a new concept in the building industry. A case study of how Six-sigma was applied to develop a new beam product is presented. Results of market studies and surveys were first discussed to reflect the voices of customers. Through the QFD (Quality Function Deployment) methodology, voices of customers were converted into quality characteristics of structural beams. Then, using Pugh matrix methodology, design concept of hollow flanged channel was derived and developed. Pilot test results showed that the developed MCO (Modular Construction Optimized) beam has superior performances and higher accuracy for fabrication reducing steel quantities.