A direct wafer bonding process necessary for GaAs-on-insulator (GOI) fabrication with high thermal annealing temperatures was studied by using PECVD oxides between gallium arsenide and silicon wafers. In order to apply some uniform pressure on initially-bonded wafer pairs, a graphite sample holder was used for wafer bonding. Also, a tool for measuring the tensile forces was fabricated to measure the wafer bonding strengths of both initially-bonded and thermally-annealed samples. GaAs/$SiO_2$/Si wafers with 0.5-$mu$m-thick PECVD oxides were annealed from $100^{circ}C;to;600^{circ}C$. Maximum bonding strengths of about 84 N were obtained in the annealing temperature range of $400{sim}500^{circ}C$. The bonded wafers were not separated up to $600^{circ}C$. As a result, the GOI wafers with high annealing temperatures were demonstrated for the first time.