The study is to find an optimum method of fixture design to ensure the machining flatness error to be minimum due to the moving cutting heat source imposed by finish face milling cutter. Through the result from cutting experiments and computational analyses, it is found that flatness error is dominantly influenced by the cutting heat. Therefore, concerning the surface flatness due to moving cutting heat source, different types of fixture layout design are presented for finish machining of relatively thin plate shaped workpieces. Meanwhile, the computational model of flatness error is included in the optimization methodology and applied in searching for fixturing support layout for the minimum flatness error. Experiments and computational analyses show the effectiveness of the optimization methodology.