The　bar　diameter　is　one　of　the　key　factor　affecting　the　bond　performance　between　GFRP　bar　and　ultra-high　performance　concrete　(UHPC).　However,　there　is　no　consensus　regarding　the　effect　of　bar　diameter　on　the　corresponding　bond　strength　in　existing　studies.　An　experimental　study　on　centric　pull-out　test　of　helically　ribbed　GFRP　bars　with　nine　different　diameters　was　conducted　to　investigate　the　effect　of　bar　diameter　on　the　bond　strength　between　GFRP　bars　and　UHPC.　The　parametric　analysis　regarding　steel　bars　and　UHPC　was　investigated　in　the　comparative　test.　For　GFRP　bars　with　diameters　ranging　from　8　mm　to　25　mm　and　an　embedment　length　of　1-2　times　the　bar　diameter,　the　bond　strength　between　UHPC　and　GFRP　bars　varies　between　26.5　MPa　and　56.2　MPa.　The　bond　strength　initially　increases　with　the　bar　diameter　in　the　range　from　8　mm　to　18　mm,　while　it　decreases　in　the　range　from　18　mm　to　25　mm.　The　relative　rib　height　Hrd　is　observed　as　the　decisive　factor　among　geometric　parameters　of　the　GFRP　bar　affecting　the　bond　strength.　In　addition,　a　correction　factor　Kr　obtained　from　the　geometric　parameters　and　experiments　results　was　proposed　to　modify　the　existing　prediction　formula　of　bond　strength.　The　prediction　of　the　proposed　formula　shows　a　good　agreement　with　the　experimental　results.