White light-emitting thin films containing Ln(3+)-doped NaYF4 nanoparticles were prepared by a simple spin-coating method. White light was generated by two different lanthanide ions, Er3+ (red and green) and Tm3+ (blue), by upconversion process under the excitation of a 980-nm laser diode. The ratio of the intensity of the three main emissions was tuned by controlling the concentration of the nanoparticles in the thin film and the concentration of the lanthanide ions in the nanoparticles. The color coordinates corresponding to emissions of different nanoparticle concentrations and with the different pump powers were investigated. When the pump power was fixed at 900 mW, the thin film with a concentration ratio of 2.5:1 emitted pure white light with coordinates of (0.333, 0.339).