In　this　study,　the　size　effect　of　the　thickness　of　the　specimen　on　the　mechanical　properties　of　UHPC　was　investigated　through　compression　tests　under　cyclic　loading.　UHPC　prisms　with　different　thicknesses　from　20　mm　to　100　mm　were　tested　under　unloading/reloading　cycles　and　the　stress-strain　curves　were　recorded　for　this　purpose.　A　constitutive　model　to　predict　the　stress-strain　response　of　UHPC　accounting　for　the　thickness　was　proposed.　It　was　found　that　UHPC　specimens　with　different　thicknesses　showed　the　same　failure　mode　of　di-agonal　shear　failure.　The　size　effect　of　thickness　was　significant　for　the　compressive　strength　but　was　negligible　for　the　peak　strain　and　elastic　modulus.　The　damage　of　the　unloading　modulus　and　reloading　modulus　at　zero　stress　only　occurred　when　the　unloading　strain　exceeded　0.8　times　the　peak　strain.　Furthermore,　the　proposed　constitutive　model　could　predict　the　stress-strain　response　for　UHPC　with　different　thicknesses　under　cyclic　compressive　loading　with　acceptable　accuracy.