This　paper　presents　an　experimental　study　that　investigates　the　effect　of　fiber　dimension　on　preventing　spalling　of　ultra-high-performance　concrete　(UHPC).　UHPCs　with　different　dimensions　of　polypropylene　(PP)　fibers　were　prepared　for　permeability　test　at　different　temperatures　and　spalling　test　under　ISO834　fire　curve.　Scanning　electron　microscope　(SEM)　observation　was　also　conducted　focusing　on　the　fiber　of　a　UHPC　sample　at　the　same　location　after　it　was　subjected　to　elevated　temperatures.　The　results　of　permeability　tests　and　spalling　tests　showed　that　longer　fibers　of　12 mm　and　18 mm　and　finer　fibers　of　28 μm　and　33 μm　were　more　beneficial　for　increasing　the　permeability　of　UHPC　at　high　temperature,　resulting　in　higher　effectiveness　on　spalling　prevention.　Permeability　measurements　indicated　a　significant　increase　occurred　at　150°C　regardless　of　fiber　dimensions;　this　increase　plays　an　important　role　in　spalling　prevention.　SEM　results　further　testify　that　this　increase　can　be　attributed　to　the　formation　of　microcracks　caused　by　expansion　of　PP　fibers.　An　aspect　ratio　of　300　is　recommend　for　spalling　prevention　of　UHPC,　when　3 kg/m3　of　PP　fibers　are　used.　©　The　Author(s),　under　exclusive　licence　to　Springer　Science+Business　Media,　LLC,　part　of　Springer　Nature　2022.　Springer　Nature　or　its　licensor　holds　exclusive　rights　to　this　article　under　a　publishing　agreement　with　the　author(s)　or　other　rightsholder(s);　author　self-archiving　of　the　accepted　manuscript　version　of　this　article　is　solely　governed　by　the　terms　of　such　publishing　agreement　and　applicable　law.