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In this study, the damaging effects of seismic stress-controlled fatigue (low-cycle and high-cycle fatigue) and seismic strain–controlled fatigue (extremely low-cycle fatigue) on Grade St-37 and St-52 steels were studied. So far, no comprehensive studies have been performed on the fatigue parameters of these two types of steels and their properties have not been compared with each other. In this study, S-N curves of grade St-37 and St-52 steels were obtained by performing rotational fatigue test (experimental stress–controlled test) on more than 80 specimens, and the behavior of these two types of steels under seismic stress–controlled fatigue was compared with each other. The cyclic void growth model (CVGM) was used to investigate the effects of seismic strain–controlled fatigue on steels. The parameters used in the CVGM for the most commonly used steels in Iran (St-37, St-52) were obtained on the basis of monotonic and cyclic loading test results (experimental strain-controlled test) on more than 50 specimens. To compare the behavior of these two types of steel and their structural moment connections, four types of connections of Grade St-37 and four types of St-52 material were modeled and examined under seismic stress–controlled fatigue and seismic strain–controlled fatigue. The results show that under seismic stress–controlled fatigue and seismic strain–controlled fatigue, St-52 steel moment connections have a better fatigue strength than St-37 steel connections. Also it can be said that under strain-controlled fatigue, bolted flange plate connections have the best performance, and under stress-controlled fatigue, welded reduced beam section connections have the best performance. [ABSTRACT FROM AUTHOR]