Polarized　upconversion　luminescence　(UCL)　of　lanthanide-doped　micro/nano-crystals　has　shown　great　promise　in　single-particle　tracking　and　super-resolution　bioimaging.　However,　because　of　the　spectral　line　broadening　and　multiple　sites　of　lanthanide　in　upconversion　particles　(UCPs),　the　crystal-field　(CF)　polarization　components　of　UCL　are　usually　undistinguishable.　Herein,　we　report　the　linearly　polarized　UCL　in　LiLuF4:Yb3+/Er3+　single　microcrystals　with　resolvable　CF　transition　lines　and　a　polarization　degree　up　to　0.82.　The　CF　levels　and　CF　transition　lines　of　Er3+,　as　well　as　their　emission　polarization　anisotropy,　are　unraveled　for　the　first　time　through　low-temperature　and　high-resolution　photoluminescence　(PL)　and　UCL　spectroscopies.　By　taking　advantage　of　the　well-resolved　and　highly-polarized　CF　transition　lines　of　Er3+,　we　demonstrate　the　application　of　LiLuF4:Yb3+/Er3+　single　microcrystals　as　anisotropic　UCL　probes　for　orientation　tracking.　These　findings　provide　fundamental　insights　into　the　polarization　anisotropy　of　UCL　in　lanthanide-doped　single　particles,　thus　laying　a　foundation　for　the　future　design　of　anisotropic　luminescent　probes　towards　versatile　applications.[Figure　not　available:　see　fulltext.].　©　2021,　Science　China　Press　and　Springer-Verlag　GmbH　Germany,　part　of　Springer　Nature.