We have studied 6H-SiC(0001)Si surfaces with and without Ni atoms on top. 6H-SiC(0001) clean surfaces were obtained by resistive heating of a SiC sample for a few minutes at about 1100 o C in UHV. Structural change of such clean 6H-SiC(0001) surface was studied by scanning tunneling microscope (STM) after heat treatment at several temperatures. Then we deposited 1-2Å of Ni on thus cleaned surface. The initial stage of the adsorption process of Ni atoms on 6H-SiC(0001) substrate was analyzed using STM to clarify Ni islands formation. Furthermore, we heated Ni(1-2Å)/6H-SiC(0001)Si specimens at 400-800 o C. After heating at 600 o C, 2√3x2√3 surface structure was observed. Structure model of this surface has been proposed.