In the present research, the photochromic MoO 3 powder with novel morphology has been synthesized via hydrothermal method, using formaldehyde as the capping agent. The formaldehyde-induced MoO 3 powder is found to have hierarchically mesoporous spherical architecture and enhanced photochromic properties. The possible growth mechanism of the MoO 3 hierarchical microstructure is discussed. And the effect of formaldehyde on the morphology and performances of MoO 3 powder has been studied by techniques of X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), as well as ultraviolet and visible spectroscopy (UV–vis).