Evolution of surface morphology of ${alpha}-Al_2O_3$ substrate was investigated as a function of annealing temperature and time. Commercial (0001) ${alpha}-Al_2O_3$ single crystal substrates were annealed in the range of $600-1000^{circ}C$ in air. At $600^{circ}C$, step-terrace structure started to be formed on the substrate. However, the surface roughness on the terrace was still considerable and a number of islands were observed on the step edges as well as the terraces. As the annealing temperature increased, the islands were absorbed into the step edges. Thus the terraces were smoother and the step edges were more straightened. Well-defined surface with a step height of 0.2 nm was formed above $900^{circ}C$. On the other hand, when the substrate was annealed at a fixed temperature of $1000^{circ}C$, the change of surface morphology was observed for the substrate annealed for 10 min. After the annealing for 30 min, the surface on which any islands could not survive was observed.