Thermal　residual　stress　generated　during　curing　is　known　to　be　detrimental　to　mechanical　performance　of　a　carbon　composite.　Fiber　prestressing　technique　has　been　developed　for　decades　to　counterbalance　these　negative　effects.　Although　there　have　been　some　achievements　in　development　of　the　prestress　mechanisms,　these　are　mainly　based　on　the　extrapolations　from　macroscopic　mechanical　characterizations,　lack　of　direct　evaluation　of　the　in-situ　in-plane　strain　or　stress　evolution　mechanisms,　in　order　to　reveal　the　impact　induced　by　different　prestressing　methods.　Here,　we　investigated　the　in-situ　strain　evolution　mechanisms　in　producing　a　prestressed　carbon　composite.　Since　clamping　of　the　uncured　prepreg　is　the　most　challenging,　both　the　strain　evolutions　with　and　without　precured　prepreg　edges　were　evaluated　to　reveal　the　underlying　mechanisms　induced　by　stress　relaxation　during　fiber　prestressing.　Mechanical　tests　in　terms　of　Charpy　impact　and　three-point　bending,　as　well　as　fractured　morphology　were　carried　out　to　evaluate　the　prestress　effects.　The　underlying　mechanisms　were　then　proposed　to　reveal　the　fundamentals　in　producing　a　prestressed　composite.　These　are　expected　to　revolutionize　industrial　production　and　applications　of　prestressed　polymeric　composites,　indicating　detrimental　mechanisms　on　precuring　strip　ends　following　conventional　prestress　procedures.　Highlights:　There　is　an　optimal　prestrain　level　to　maximize　properties　of　a　composite.　Internal　strain　development　is　dependent　on　the　fiber　clamping　methods.　Stress　relaxation　is　beneficial　to　induce　compressive　stress　within　a　composite.　In-situ　strain　evolution　mechanics　is　revealed　for　prestressed　carbon　composite.　©　2024　Society　of　Plastics　Engineers.