Morphological evolution of on-axis 6H- and 4H-SiC{0001} surfaces was systematically investigated under a "pure" thermal annealing condition by using a newly developed graphite-free vacuum furnace. Without the effect of ambient gas pressure, the surfaces were treated at high temperatures (1300 degreesC - 1900 degreesC) in high vacuum (10(-5)-10(-4) Pa) with a fast heating/cooling rate. The surface morphological changes of these surfaces by 15 min annealing were characterized by ex situ atomic force microscopy (AFM). Well-defined step bunching phenomena were observed on the Si faces of both 4H and 6H in a wide temperature window (1300-1600degreesC). The surface evolutions on the Si-faces revealed that a step-step impulsion effect accelerating the step bunching on the 4H surface is larger than the case of 6H. The most pronounced bunched steps appeared at around 1600degreesC, which is just below the graphitization temperature for both 6H and 4H surfaces. In case of C-faces, the bunched step was not observed in this experiment, although the carbon nanotube (CNT) mesh features appeared above 1600degreesC. These differences in the surface evolutions are considered to be attributed to a polytypism-mediated surface stability during the annealing.