Efficient up/down conversion (UC/DC) thin films with high transmittance were prepared by an anisole dissolved polymethyl methacrylate (PMMA) solution mixed with beta-NaYF4: 5% Er, 10% Yb (UCNPs) and beta-NaYF4: 10% Tb, 1% Yb (DCNPs) nanocrystals, which required no postdeposition heat treatment. The full embedment of nanoparticles in the PMMA matrix was confirmed by atomic force microscopy, and the films were dense and uniform with high transmittance. Both UCNPs and as-prepared films exhibited strong UC emission bands in the green and red region. The relative intensities of green to red emissions in the films were enhanced compared with UCNPs because of the surface passivation by PMMA. Under excitation of 355 nm laser, both DCNPs and as-prepared films exhibited down conversion luminescence and no obvious intensity difference was observed between nanoparticles and the films. By tape test, the conversion films were proved with a strong adhesion due to the protection of PMMA. The results suggested that PMMA is a good matrix for fabrication of efficient UC/DC films with high transmittance and good adhesion.