By embedding NaYF 4 :Yb 3+ ,Tm 3+ nanocrystals into the top cladding layer of a resonant waveguide grating structure, we demonstrate that the upconversion fluorescence of Tm 3+ ions can be greatly enhanced, by a factor of up to 10 4 . The resonant waveguide grating structure consists of an SU8 bottom layer with sinusoidal grating morphology coated with a thin TiO 2 waveguide layer and then covered with a poly(methyl methacrylate) cladding layer doped with NaYF 4 :Yb 3+ ,Tm 3+ nanocrystals. The giant enhancement of the upconversion fluorescence is achieved first by coupling the excitation light with a guided mode of the resonant waveguide grating structure and then the fluorescent light with a second guided mode. Our numerical simulation results obtained by rigorous coupled-wave analysis indicate that the electric field of the incident light is strongly enhanced near the interface of the TiO 2 layer and the poly(methyl methacrylate) layer at guided mode resonance, and this is the major effect of the observed enhancement of the upconversion fluorescence of the nanocrystals. The resonance between the fluorescent emission and the waveguide structure further enhances the intensities of the fluorescent signal. We also find that the lifetime of upconversion fluorescence at 480 nm wavelength from the rare-earth nanocrystals is reduced about 1.34-fold when both excitation and extraction resonance occurs in the waveguide structure.