Is there any difference or what difference is in fatigue life between high-cycle fatigue tests of steel structures at the loading with low， medium， and high frequencies has always been a question for experimenters to find an answer to. Comparative analysis of fatigue test data （including tests conducted by the authors of the present paper and other scholars） was performed for structural steel as well as weldment at loadings between low frequency （below 30Hz） and medium-high frequency （around 100Hz）. The effects of low and medium-high frequency loadings on fatigue life was investigated. The fatigue tests dealt with high strength steel Q460 and its butt-welded joints， as well as butt-welded joints of normal strength steel Q345. It was found that there was substantial difference in fatigue life between low and medium-high frequency loadings. The former had a longer fatigue life or higher fatigue strength than the latter， whether it was steel or butt-welded joints. The mechanism for this phenomenon was also explained. It is suggested that for engineering steel structures subjected to low frequency fatigue loadings， fatigue tests should be conducted using low frequency fatigue testing machine. It may significantly underestimate fatigue life if a fatigue testing machine with a frequency higher than low frequency is used.