The　effect　of　sulfate　attack　(SA)　on　the　strength　of　ultrahigh-performance　concrete　(UHPC)　made　with　different　replacements　(0%,　50%　and　100%　by　weight)　of　natural　fine　aggregate　with　recycled　fine　aggregate　(RFA)　was　investigated.　The　UHPC　samples　were　soaked　in　10%　sodium　sulfate　solution　for　0-180　days　(SA　duration).　The　sulfate　ion　concentration,　flexural　strength　and　compressive　strength　of　the　UHPCs　were　investigated.　The　microstructure　of　the　UHPCs　before　and　after　SA　was　analysed　using　a　multi-technique　approach　(analysis　of　interfacial　transition　zones　(ITZs),　X-ray　diffraction,　scanning　electron　microscopy-energy-dispersive　X-ray　spectroscopy　and　mercury　intrusion　porosimetry).　The　results　showed　that,　with　an　increase　in　SA　duration,　the　compressive　strength　of　the　UHPC　made　with　RFA　increased.　The　generated　gypsum　and　ettringite　refined　the　microstructure　by　converting　harmful　pores　(＞20　nm)　into　harmless　pores　(＜20　nm)　in　the　UHPC　matrix.　For　the　same　SA　duration,　the　sulfate　ion　concentration　of　UHPC　increased　with　an　increase　in　RFA　content　due　to　more　ITZs,　pores　and　microcracks　introduced　by　the　RFA.　Strength　development　models　for　UHPC　with　different　RFA　contents　under　SA　were　developed.