One　of　the　key　mechanical　problems　of　strengthening　RC　beams　with　carbon　fiber　laminate　(CFL)　is　the　CFL-concrete　interfacial　fatigue　properties.　Fatigue　and　fracture　mechanics　method　is　used　to　develop　a　calculation　formula　of　stress　intensity　factor　(SIF)　for　the　interfacial　fatigue　cracks.　Based　on　theoretic　analysis　and　a　serial　of　fatigue　tests,　the　propagation　law　and　the　predicting　method　of　the　interfacial　fatigue　cracks　are　discussed.　The　main　results　show　that:　1)　The　SIF＇s　value,　a　determined　factor　of　the　fatigue　crack　propagation　life,　is　a　direct　ratio　to　that　of　CFL　strain.　When　the　CFL　strain　range　is　below　the　value　of　a　＂strain　threshold＂,　the　interfacial　fatigue　cracks　will　not　propagate;　2)　The　process　for　the　initiation　and　stable　or　unstable　growth　of　the　interfacial　cracks　is　observed.　The　interfacial　fatigue　crack　propagation　life　of　the　RC　beams　strengthened　with　CFL　under　cyclic　bending　loads　can　be　precisely　predicted　by　using　a　similar　approach　as　Paris　formula.