With the constructed thermal analysis database, optimization for a vacuum furnace was performed to improve its energy efficiency. Insulator thickness and heater size were selected as the design variables and the heat flux requirement was used as the objective function. Four different initial values yielded the same converged optimization showing a global solution. The optimal result showed a reduction of 3~19% for the energy requirement in comparison with the initial designs. Also, optimization could be possible within one minute due to the efficiency of the constructed analysis database. When the manufacturing cost is accounted for along with the heat flux requirement, alternative optimizations are also possible, which can be useful guidelines to a manufacturer.