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Metasurfaces are two-dimensional optical elements capable to manipulate phase, polarization and propagation direction of light. The concept of metasurfaces was employed to enhance light-matter interactions and nonlinear effects at the nanoscale. As a result, different phenomena such as all-optical switching with a high modulation depth, an analogue of electromagnetically induced transparency and third-harmonic generation were realized by dielectric metasurfaces. A new concept that enriches the variety of effects and further boosts the nonlinear effects, in particular, corresponds to using of dielectric metasurface with high-Q resonances. However, the idea of high-Q resonances with designer dispersion has not been utilized to additionally enhance the nonlinear optical effects. In this contribution, we experimentally observe the enhancement of third-harmonic generation signal in a Si-based metasurface with a certain dispersion band design – “flat-band” dispersion. With such metasurface the extended range of k-vectors of incident light perfectly matches the position of metasurface resonance. Previously, the same optical states were demonstrated in photonic crystals. Here, we, for the first time to our knowledge, demonstrate a flat-band dispersion in high-Q resonant metasurface by angle-resolved spectroscopy and third-harmonic generation spectroscopy. We specifically design and experimentally realize silicon metasurface with different curvatures of band dispersion. The enhanced THG signal is detected when the dispersion band becomes flat that leads to trapping of the additional angular components of incident focused beam.