This study is concerned about the optimal design of the main frame in a moper. Mopper is widely used for cleaning in the fields such as airport, subway, train station, large shopping mall, department store, hospital, government buildings, school, industrial building floor, etc. Especially, main frame in mopper are stressed by high, mostly loads. Nevertheless, design of main frame is dependent on experience, and it will cause reliability reduction and cost increase. This research purpose is to propose and verify a method to achieve higher product quality, lower costs, and far less physical testing. In this study, design parameters used for optimization are each thickness of 4 square pipes and 1 plate in the main frame. The result after optimization showed that the weight of main frame reduced to maximum 6.5% when compared with the initial design in main frame.