Silver nanoparticles are added into Yb3+ and Er3+ co-doped NaYF4 up-conversion materials using co-sintering process. Changes in diffraction intensity and surface morphology are inspected by X-ray diffraction and scanning electron microscope measurement, respectively. The optical absorption and Photoluminescence spectra are measured using UV/visible spectrophotometer and photoluminescence measurements. By optimizing the quantity of Ag nanoparticles, we obtain obvious enhancements of the photoluminescence intensities of Yb3+ and Er3+ co-doped NaYF4 materials, which shows a 28% enhancement in 300—800 nm wavelength range and a 55% enhancement at 544 nm. Different mechanisms are proposed for light quenching and surface plasmon enhanced absorption with Ag nanoparticles adjustment.