This　paper　investigated　mechanical　properties　of　ultra-high　performance　concrete　(UHPC)　after　exposure　to　elevated　temperatures　ranging　from　200　degrees　C　to　800　degrees　C.　Two　casting　methods　were　applied　to　prepare　UHPC　specimens　containing　randomly　distributed　or　aligned　steel　fibers.　X-ray　diffraction　(XRD),　mercury　intrusion　porosimetry　(MIP)　and　scanning　electron　microscope　(SEM)　were　utilized　to　evaluate　the　microstructure　degradation　of　UHPC　with　high　temperatures.　Experimental　results　indicated　that　compressive　strength,　flexural　strength　and　toughness　were　significantly　reduced　by　83.4%,　82.5%　and　84.6%　for　UHPC　specimens　with　randomly　distributed　steel　fibers　after　exposure　to　800　degrees　C.　The　alignment　of　steel　fibers　can　positively　alleviate　the　degradation　of　mechanical　properties　of　UHPC,　inducing　an　increase　of　33.8%,　72.4%,　43.9%　and　58.8%　for　the　residual　compressive　strength,　flexural　strength,　toughness　and　deflection　at　post-cracking　peak　point,　respectively.　According　to　microstructure　measurements,　cement　hydration　products　were　badly　decomposed　by　high　temperature　exposures　with　the　occurrence　of　coarsening　pore　structure　in　cement　paste　and　worsening　the　interface　between　cement　paste　and　steel　fibers,　subsequently　causing　the　mechanical　performance　degradation　of　UHPC　specimens.　Although　the　tensile　strength　of　fibers　was　notably　reduced　by　high　temperatures,　the　fibers　still　performed　bridging　effects　on　cracks　in　UHPC.　This　is　the　main　reason　that　the　fiber　alignment　can　significantly　improve　the　residue　mechanical　properties　of　UHPC　after　exposure　to　high　temperatures.　Therefore,　the　high　temperature　resistance　of　UHPC　can　be　improved　by　optimizing　the　orientation　of　steel　fibers.