The structure change of SrTiO3 (111) polar surface was studied as a function of atmosphere. Atomic force microscopy (AFM) and coaxitial impact-collision ion-scattering spectroscopy (CAICISS) were used to study the surface structure of the (111) polar surfaces of SrTiO3 single crystals. From the result of AFM observation, the polar surfaces annealed in oxygen gas had a trench structure, and that annealed in argon gas formed large triangular terraces. The surface annealed in air, i.e. in intermediate oxygen partial pressure, consisted of complicated self-similar triangles with different sizes, forming a fractal shape called Sierpinski's gasket. The time of flight spectra of CAICISS suggested that a Ti4+ plane was more stable than a SrO34- plane on the substrate annealed in oxygen gas, and the Ti4+ and SrO34- planes almost equally existed on the substrate surface annealed in argon gas. Molecular dynamic calculations at constant temperature and pressure were applied for various SrTiO3 (111) polar surfaces.