The embedded atom method is used to study the structure stability of gold nanobelt. The Au nanobelts have a rectangular cross-section with 〈100〉 orientation along the x-, y- and z-axes. Free surfaces are used along the x- and y-directions, and periodic boundary condition is used along z-direction. The simulation is performed at different temperatures and cross-section sizes. Our results show that the structure stability of the Au nanobelts depends on the nanobelt size, initial orientation, boundary conditions and temperature. A critical temperature exists for Au nanobelts to transform from initial 〈100〉 nanobelt to final 〈110〉 nanobelt. The mechanism of the reorientation is the slip and spread of dislocation through the nanobelt under compressive stress caused by tensile surface-stress components.