Many　bridge　structural　components　are　subjected　to　repetitive　vehicle　load　and　temperature　gradient　action.　The　resulting　cyclic　tensile　stresses　within　the　structures　could　cause　premature　fatigue　failure　of　concrete,　dramatically　impairing　structural　components’　durability　and　sustainability.　Although　substantial　knowledge　of　fatigue　properties　on　low-strength　pavement　concrete　and　high-strength　structural　concrete　has　been　obtained,　research　on　the　most　widely　used　normal-grade　ordinary　concrete　in　bridge　engineering　is　still　ongoing.　Therefore,　a　four-point　bending　fatigue　test　of　97　C50　concrete　specimens　under　a　constant　amplitude　sinusoidal　wave　was　conducted　in　the　laboratory,　the　flexural　fatigue　behavior　of　plain　and　reinforced　concrete　specimens　was　studied,　and　the　cyclic　deformation　evolution　of　concrete　under　fatigue　loading　was　obtained.　The　empirical　fatigue　S-N　equations　of　concrete　with　a　failure　probability　p　of　0.1~0.5　were　derived　through　statistical　analysis　of　the　test　results.　The　fatigue　life　of　the　tested　specimens　exhibited　a　two-parameter　Weibull　distribution.　In　addition　to　the　maximum　stress　level　Smax,　the　stress　ratio　R　is　also　a　key　factor　affecting　the　flexural　fatigue　life　of　concrete　N.　The　semi-logarithmic　and　logarithmic　equations　were　almost　identical　at　the　tested　stress　levels,　the　latter　predicting　longer　fatigue　life　for　Smax　＜　0.70.　The　restraining　effect　from　steel　reinforcement　slightly　lengthened　the　concrete’s　fatigue　cracking　initiation　life.　The　insight　into　concrete　flexural　fatigue　properties　from　this　study　not　only　contributes　to　a　better　understanding　of　structural　concrete,　but　also　provides　a　basis　for　the　practical　evaluation　of　concrete　or　composite　bridge　decks.　©　2023　by　the　authors.