Monoclinic gallium oxide nanosheets were synthesized on a large scale through a simple gas reaction method. They were characterized by means of X-ray powder diffraction (XRD), field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDS), high-resolution transmission electron microscopy (HRTEM), and Raman spectroscopy. FE-SEM images showed that the product was mainly composed of nanosheets. XRD, EDS and Raman spectra indicated the nanosheets were a monoclinic gallium oxide structure. An HRTEM lattice image revealed that the nanosheets are nearly uniform in structure and single crystalline.