This　study　conducted　a　thorough　investigation　on　the　combined　effects　of　fine　aggregate　(FA)　size,　steel　fiber,　and　polypropylene　(PP)　fiber　on　the　spalling　behavior　and　mechanical　properties　of　ultra-high-performance　concrete　(UHPC)　at　high　temperature.　FAs　with　0.6,　2.36,　and　4.75　mm　were　incorporated　with　steel　fibers　or　PP　fibers　in　UHPC.　Test　results　showed　that　the　synergistic　enhancement　in　spalling　prevention　of　UHPC　at　high　temperature　was　only　found　in　the　combination　of　PP　fiber　and　large-sized　FA.　Large-sized　FA　not　only　increased　the　fraction　of　microcracks　but　also　enhanced　their　connectivity　in　UHPC　with　PP　fibers,　thus　increasing　the　permeability　and　improving　the　spalling　resistance　at　high　temperature.　This　reduced　the　required　PP　fiber　content　for　spalling　prevention.　Besides,　steel　fibers　and　large-sized　FAs　had　a　combined　negative　effect　on　mechanical　properties　above　600　degrees　C,　resulting　in　even　lower　mechanical　properties　at　900　degrees　C　compared　to　UHPC　without　any　fiber　and　UHPC　with　PP　fibers.　Microstructural　observation　also　found　that　the　degradation　of　steel　fibers　and　microcracks　generated　by　expansion　of　aggregate　both　severely　damaged　the　microstructures　of　UHPC　at　900　degrees　C.　By　contrast,　adding　PP　fibers　reduced　compressive　strength　of　UHPC　below　600　degrees　C　due　to　the　voids　left　by　the　decomposition　of　PP　fibers,　but　it　did　not　affect　compressive　strength　at　900　degrees　C,　as　the　cracks　in　the　matrix　was　enlarged,　which　reduced　the　negative　effect　of　PP　fibers.