The in situ polymerization method was applied to synthesize bulk nanocomposites consisting of hydrophobic NaYF(4):Yb, Er (Tm) nanoparticles as the filler and polystyrene (PS) as the host material. The oleic acid stabilized NaYF(4):Yb, Er (Tm) nanoparticles and NaYF(4):Yb, Er (Tm)/PS nanocomposites have been well characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM), the thermogravimetric analysis (TGA), upconversion photoluminescence spectra and luminescence decays. The well-crystallized NaYF(4):Yb, Er (Tm) nanoparticles are spherical with a mean diameter of 40 nm. NaYF(4):Yb, Er/PS and NaYF(4):Yb, Tm/PS nanocomposites exhibit strong green and blue upconversion photoluminescence upon 980 nm laser excitation, due to the integration of luminescent NaYF(4):Yb, Er and NaYF(4):Yb, Tm nanoparticles, respectively. These nanocomposites can be potentially used as 3D display materials.