This　study　aims　to　analyze　the　various　fiber　volume　fractions　(1%,　2%　and　3%)　and　reinforcement　diameters　(16,　18　and　20　mm)　on　the　tension　stiffening　mechanisms　and　effect　of　ultra-high　performance　concrete　(UHPC)　through　the　axial　tensile　test　of　reinforced　UHPC　(RUHPC).　The　results　reveal　that　the　tensile　response　of　RUHPC　can　be　categorized　into　four　stages:　elastic,　cracking,　crack　development　and　post-yield.　Both　fiber　volume　fraction　and　reinforcement　diameter　significantly　influence　crack　development　and　distribution,　with　distinct　patterns　observed　in　cracking　load,　equivalent　stiffness,　yield　capacity,　and　toughness.　The　study　further　analyzes　the　mechanisms　by　which　fiber　volume　fraction　and　reinforcement　diameter　affect　the　tension　stiffening　of　RUHPC　at　various　stages.　Additionally,　a　tension　stiffening　characterization　model　and　a　UHPC　tensile　constitutive　model　were　proposed,　effectively　predicting　the　tension　stiffening　behavior　of　UHPC　materials　and　components　under　varying　fiber　volume　fractions　and　reinforcement　diameters.