$eta$-Ga$_2$O$_3$ nanobelts and nanoparticles were synthesized from mechanically ground GaN powders with thermal annealing in a nitrogen atmosphere and an oxygen atmosphere, respectively. The study of field emission scanning electron microscopy (FESEM) on the microstructures of nanomaterials revealed that the nanobelts synthesized in the nitrogen atmosphere are with the range of 20~1000nm width and 10 ~100nm thickness, and that nanomaterials are nanoparticles with 20~50nm radius obtained by thermal annealing in an oxygen atmosphere. The crystal structure of the $eta$-Ga$_2$O$_3$ nanobelts and nanoparticles was in this study investigated by X-ray diffractometer (XRD) and high-resolution transmission electron microscope (HRTEM). The formation processes of the nanobelts and nanoparticles will be discussed in this paper