In　this　chapter,　we　propose　a　new　mechanical　model　describing　the　propagation　of　fatigue　short　cracks.　The　crack　propagation　can　be　divided　into　two　stages,　i.e.,　stage-I　is　the　initiation　of　microstructurally　short　cracks,　and　stage-II　is　the　propagation　of　physically　short　cracks.　In　stage-I,　the　grain　size　can　be　replaced　by　the　maximum　nondamaging　fatigue　crack　size　a1　which　can　be　predicted　by　basic　mechanical　properties.　The　decelerating　growth　rate　decreases　to　zero　when　the　crack　reaches　a1　in　stage-I.　Combined　with　the　basic　laws　of　the　fatigue　short　crack　propagation,　the　relations　of　physically　short　crack＇s　initiation/propagation　rates　to　material＇s　mechanical　properties,　as　well　as　the　crack　length,　the　stress　and　the　strain　can　be　established.　By　adopting　the　derived　relations,　fatigue　lifetime　of　short　cracks　can　be　successfully　determined　by　basic　mechanical　properties　only.　Our　relations　uncover　that　the　essence　of　Manson-Coffin　equation　reflects　the　mechanism　of　the　short　fatigue　crack＇s　initiationpropagation.　Predictions　of　fatigue　lifetime　using　our　relations　are　compared　with　the　results　of　well-known　experiments.　Good　agreements　are　found　in　many　aspects,　such　as　fatigue　lifetimes,　coefficients　and　exponents　in　the　relations,　especially　for　short　cracks　around　10　micrometers.　Predictions　on　the　short　crack　propagation　rates　are　also　compared　for　16　types　of　carbon　steels.　Satisfactory　consistency　shows　that　the　relations　have　wide　applicability.　©　2012　Nova　Science　Publishers,　Inc.　All　rights　reserved.