Hybrid　composites　made　of　metal-organic　frameworks　(MOFs)　and　lanthanide-doped　upconversion　nanoparticles　(UCNPs)　have　attracted　considerable　interest　for　their　synergistically　enhanced　functions　in　various　applications　such　as　chemical　sensing,　photocatalysis,　anticounterfeiting　and　nanomedicine.　However,　precise　assembly　of　MOF/UCNP　hybrid　composites　with　tunable　morphologies　remains　a　challenge　due　to　the　lack　of　effective　synthetic　methods　and　fundamental　understanding　of　the　growth　mechanisms.　Herein,　we　propose　a　modulator-directed　assembly　strategy　to　synthesize　a　series　of　ZIF-8@UCNP　composites　(ZIF-8　=　zeolitic　imidazolate　framework-8).　The　UCNPs　densely　paved　on　the　surface　of　ZIF-8　microcrystals　and　endowed　the　composites　with　intense　upconversion　blue　emission,　which　were　verified　by　steady-state/transient　photoluminescence　(PL)　spectroscopy　and　single-particle　imaging.　Ethylenediamine　(EDA)　was　firstly　used　as　a　modulator　to　fine-tune　the　predominant　MOF　facets　and　realized　distinct　morphologies　of　the　composites.　By　adjusting　the　concentration　of　EDA　from　0　to　25　mmol/L,　the　morphology　of　the　ZIF-8@UCNP　composites　was　tuned　from　rhombic　dodecahedron　(RD)　to　truncated　rhombic　dodecahedron　(TRD),　cube　with　truncated　edges　(CTE),　cube,　and　finally　a　unique　form　of　interpenetration　twins　(IT).　The　nucleation　and　growth　process　of　the　ZIF-8@UCNP　composites　was　monitored　by　time-dependent　scanning　electron　microscopy　(SEM)　images　and　the　formation　mechanism　was　thoroughly　revealed.　Furthermore,　we　demonstrated　that　the　strategy　for　assembly　of　morphology-controllable　ZIF-8@UCNP　composites　was　generally　applicable　to　various　UCNPs　with　different　sizes　and　shapes.　The　proposed　strategy　is　expected　to　open　up　new　avenues　for　the　controllable　synthesis　of　MOF/UCNP　composites　toward　diverse　applications.　[Figure　not　available:　see　fulltext.]　©　2022,　Tsinghua　University　Press.