The　effect　of　metakaolin　(MK)　and　magnesium　oxide　(MgO)　on　the　flexural　strength　of　ultra-high　performance　concrete　(UHPC)　was　investigated.　The　0-15　wt%　ordinary　Portland　cement　in　UHPC　was　replaced　with　MK　and　MgO　(MM)　with　a　weight　ratio　of　3:2.　The　fluidity　and　strength　of　UHPC,　the　pullout　performance　of　a　single　steel　fibre　in　UHPC　matrix　and　the　microstructure　(X-ray　diffraction,　thermogravimetry,　29Si　and　27Al　nuclear　magnetic　resonance,　microhardness,　scanning　electron　microscopy-energy-dispersive　X-ray　spectroscopy　and　mercury　intrusion　porosimetry)　of　UHPC　were　studied.　Results　show　that　the　flexural　strength　of　UHPC　initially　increases　and　then　decreases　with　an　increase　in　MM　replacement　ratios.　UHPC　with　10%　MM　at　the　age　of　60　days　have　the　highest　flexural　strength,　which　is　33.8%　higher　than　that　of　UHPC　without　MM.　When　MM　replacement　ratios　increases　from　0　to　10%,　MM　promotes　the　formation　of　C-(A-)S-H　and　LDHs.　The　addition　of　MgO　increases　the　pH　value,　resulting　in　more　Al　leaching　from　MK,　which　favours　Al　uptake　by　C-(A-)S-H.　MM　also　improves　the　densification　of　UHPC　matrix　and　the　bond　properties　between　fibres　and　UHPC　matrix.　However,　excessive　MM　(＞10%)　exhibits　a　stronger　clinker　dilution　effect,　and　some　of　the　cement　particles　are　covered　by　MM.　Therefore,　the　flexural　strength　of　UHPC　decreases.