Mechanical　behavior　of　ultra-high　performance　concrete　(UHPC)　using　recycled　fine　aggregate　(RFA)　was　experimentally　investigated.　It　was　found　that　the　compressive　and　flexural　strength　and　tenacity　of　UHPC　decreased　as　more　RFA　was　introduced,　but　were　enhanced　by　the　autoclaved　curing.　The　total　length　of　the　diverse　types　of　interfacial　transition　zones　(ITZs)　was　found　to　increase　along　with　the　growth　of　RFA　replacement　ratios;　the　quality　of　ITZs　was　improved　by　the　autoclaved　curing.　This　paper　integrated　the　geometric　and　mechanical　properties　of　ITZs　and　the　other　material　phases　on　the　micro　scale　to　propose　two　ITZ　parameters,　psi(1),　psi(2),　and　a　more　comprehensive　microstructural　parameter,　psi(3).　Based　on　correlation　analysis,　psi(3)　was　proved　to　be　more　reliable　in　describing　the　key　microstructural　features　of　UHPC　prepared　with　RFA,　and　also　in　quantitatively　revealing　the　mechanism　for　the　mechanical　behaviour　of　UHPC　on　the　macro　scale,　regardless　of　the　RFA　dosages　or　curing　conditions.　(C)　2018　Elsevier　Ltd.　All　rights　reserved.