The authors synthesized a kind of upconversion nanocrystals NaYF4:Yb3+,Er3+ via the hydrothermal assisted homogeneous precipitation method,and then the nanocrystal was coated with silica. The SEM image demonstrated that the as-prepared samples were uniform in size distribution with ca. 25 nm before and ca. 250 nm after silica coating,respectively. The upconversion spectra and photoluminescence lifetime measurement showed that the silica shell had hardly effect on the properties of fluorescence of the NaYF4:Yb3+,Er3+ nanocrystals. At the same time,the naked eye-visible green upconversion fluorescence pattern was acquired from the as-prepared upconversion nanoparticles in the PBS buffer (2 wt%) excited by 980 nm laser at room temperature. These water-soluble nanoparticles were linked to the antibodies using the coupling reagents glutaraldehyde. The circular dichroism (CD) spectra of antibody and upconversion nanoparticles-antibody conjugates were very similar to each other,indicating that the secondary structure of antibody remained largely intact after the conjugation. Finally,antigen-antibody recognition reaction was performed on the surface of a silicon slide. The immunofluorescence in vitro indicated that the upconversion nanoparticles-antibody bioconjugates had excellent species-specific detection ability with hardly non-specific binding. Based on the present results,it is anticipated that the silica-coated upconversion nanoparticles are suitable for use as biolabeling materials.