The photocatalytic activities of nonpolar crystal facets are different from those of polar crystal facets. It is important to directly observe photocatalytic activities of the photocatalysts with the different facets. ZnO nanoparticles are synthesized through a sol-gel method and studied by in-situ vacuum Fourier Transform infrared (FTIR) spectroscopy for photo-induced decomposition of formaldehyde. It is found that the annealing of ZnO nanoparticles results in redistribution of crystal facets with the partial transformation from polar facets to nonpolar ones, which is evidenced by X-ray diffraction (XRD), Field-emission scanning electron microscopy (FESEM) and in-situ vacuum Fourier Transform infrared (FTIR) spectroscopy. The photocatalytic activities of ZnO nanoparticles with different crystal facets are characterized by the evolution rate of IR absorption band of the adsorbates under UV light irradiation. Compared with the nonpolar crystal facets on ZnO nanoparticles, the polar crystal facets exhibit higher photocatalytic activities toward CH2O decomposition, revealing that the photocatalytic activity can be strongly affected by the crystal facet polarities.