Water-soluble BaYF5:Yb3+/Er3+ nanoparticles with the surface functionalized by a layer of poly(acrylic acid) (PAA) were synthesized via a facile one-step PAA-assisted hydrothermal method. Bovine serum albumin (BSA) protein was conjugated with BaYF5:Yb3+/Er3+ upconversion nanoparticles via free carboxylic acid groups on the surface of nanoparticles. The final products were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), infrared (IR) spectrophotometry, ultraviolet (UV) spectrophotometry and photoluminescence spectroscopy (PL). The XRD results showed that PAA-capped BaYF5:Yb3+/Er3+ upconversion nanoparticles could be obtained via a PAA assisted hydrothermal process with the pH value of 8 at 200 degrees C for 24 h. The TEM results showed that the morphology of BaYF5:Yb3+/Er3+ nanoparticles was spherical particles with an average diameter of about 4 nm. The IR and UV spectra showed that BSA has been conjugated with BaYF5:Yb3+/Er3+ up-conversion nanoparticles. The luminescence properties of BaYF5:Yb3+/Er3+ up-conversion nanoparticles were also studied. The luminescence properties of the products suggest that BaYF5:Yb3+/Er3+ upconversion nanoparticles have promising applications for labels in biological assays.