This paper deals with improvement of superheated steam temperature fuzzy control of high-pressure part in once-through boiler. The improvement of fuzzy control quality is based on optimization of fuzzy controller parameters. The optimization is based on minimization criteria. Optimized fuzzy parameters are function of high pressure steam temperature. More possibilities of optimization are presented in the paper. Fuzzy control properties are verified by simulation experiments within operating mode change, response to disturbance and change of system dynamics. Responses of fuzzy control are compared before and after the optimization.