The etching of a (4H, 6H)-SiC{0001}substrates with an electrochemical method was studied. The dependence of etching rate on the polytype, surface polarity and pH value are measured quantitatively. Under an optimum etching condition, the surface roughness of Si face could be improved to 0.9 nm (4H) and 0.5 nm (6H) compared with as-received substrates. Under other etching conditions, crystal defects such as micropipes and triangle shape pits were observed for epitaxial surfaces. Based on these results, etching mechanisms of alpha-SiC in the electrochemical method are discussed.