This　paper　investigated　pore　pressure　development　of　ultra-high　performance　concrete　(UHPC)　included　various　polymer　fibers,　i.e.,　linear　low-density　polyethylene　(LLDPE),　ultra-high　molecular　weight　polyethylene　(UHMWPE),　polypropylene　(PP),　polyester　(PET),　and　polyamide　(PA)　fibers.　Temperature　and　pore　pressure　were　measured　simultaneously　at　different　depths　of　UHPC　specimens　subjected　to　one-dimensional　heating.　It　was　found　that　the　PP　and　PA　fibers　prevented　spalling　of　UHPC　by　enhancing　moisture　migration,　which　resulted　in　the　development　of　pore　pressure　in　the　deeper　region　of　the　specimens.　The　moisture　migration　in　UHPC　with　LLDPE　fibers　caused　spalling　of　a　layer　of　concrete　in　a　deep　region　of　specimen.　UHMWPE　fibers　did　not　affect　pore　pressure　development　and　spalling　resistance　of　UHPC　significantly,　while　with　PET　fibers,　the　pore　pressure　of　UHPC　raised　sharply　due　to　inadequate　moisture　migration,　leading　to　spalling　of　a　whole　layer.　Instead　of　melting　polymer　fibers　and　empty　channels　left,　microcracks　created　by　the　fibers　were　responsible　for　releasing　vapor　pressure　and　spalling　prevention.　Fibers　with　high　thermal　expansion　between　100　and　200 °C　are　recommended　for　spalling　prevention　of　UHPC.