Enhancement　of　upconversion　luminescence　is　imperative　for　the　applications　of　upconversion　nanocrystals　(UCNs).　In　this　work,　we　investigated　the　upconversion　luminescence　enhancement　of　NaYF4:Yb/Er　by　Mo3+　ion　doping.　It　was　found　that　the　upconversion　luminescence　intensities　of　the　green　and　red　emissions　of　UCNs　co-doped　with　10　mol%　Mo3+　ions　were　enhanced　by　6　and　8　times,　respectively.　This　enhancement　offers　a　potential　increase　in　the　overall　detectability　of　upconversion　nanocrystals.　HeLa　cell　imaging　using　NaYF4:Yb/Er/Mo　as　luminescent　probes　showed　bright　upconversion　fluorescence.　Moreover,　the　Mo3+　doping　endowed　the　UCNs　with　excellent　paramagnetic　behavior.　It　is　expected　that　the　as-prepared　UCNs　with　a　high　upconversion　luminescence　and　excellent　paramagnetic　properties　could　be　promising　bi-functional　nanoprobes　for　sensitive　multi-modal　bioimaging　and　other　optical　applications.