Feedwater heater tubes with local wall-thinning defects are often found in the heat exchangers of thermal power plants. One of the most typical local wall-thinning shapes is circumferential hoop flaw defect. Determining the maximum stress acting on the tube with a circumferential defect is essential to ensure the safe operation of the heat exchange system before the tube can be failure due to stress concentration at the local thinning. This paper introduces a theoretical and numerical analysis of the maximum principal stress on the feedwater heater tubes with a circumferential hoop flaw defect under pressure and thermal loadings. The effect of three dimensions of this defect on the maximum principal stress is also studied. Using finite element analysis and regression, analytic equation for the maximum principal stress on the inner surface of a tube with a circumferential hoop flaw defect is proposed. It is found that the maximum principal stress occurs on the inner surface of the tube. The depth of the defect is the main factor influencing the maximum principal stress. The longitudinal length and the circumferential angle parameter of the defect also have effect on the maximum principal stress.