In order to improve the reliability of PWR steam generators, we have performed research to improve the tubing material and tube-support-plate configuration, based on our wide operating experience, and have developed and verified TT Alloy 690 as the optimum tubing material and the EEC type tube-support design. In the research, we have studied the metallurgical mechanism of the alloy to improve its corrosion resistance, evaluated corrosion susceptible region quantitatively, estimated the actual environment in a steam generator and confirmed the reliability by a model boiler test over a long period. It has been verified that the steam generator with the latest design has higher reliability with respect to the corrosion resistance of tubes.