This work reports the efficient structural restoration of defective graphene oxide (GO) to a crystalline graphene by an ultrahigh temperature process at around 1800 °C achieved by a solar furnace. The GO samples were treated at high temperature by irradiating concentrated sunlight and focusing it on the sample under an inert nitrogen environment at atmospheric and reduced pressure. The structural restoration of GO was analyzed by Raman spectra, and the features of their D- and 2D-bands were remarkably improved at ultrahigh temperatures. The restoration was induced not by a photochemical reaction but dominantly by a thermally stimulated reaction. The process under reduced pressure gives rise to significantly better features in the Raman spectra than that of the atmospheric condition. This tendency shows that a trace amount of impurities contained in pure nitrogen gas are not negligible and attack the GO surfaces to induce considerable defects. These results indicate the superiority of the ultrahigh temperature process at reduced pressure for efficient GO restoration and the formation of highly crystalline graphene.