A nonlinear optimal regulator with simple calculation is proposed for temperature control of a CSTR. The method is based on the expansion of a regulator for a single-variable affine system to a two-variable system, and it is smoothly combined with bang-bang control. The proposed method does not require matrix calculation or solution search, and it can be implemented in a relatively traditional computer. In experiments with a simulated-reactor system, this method showed excellent performance for set-point change. disturbance and tracking for optimal trajectory.