Contactless resistivity mapping, scanning electron microscope (SEM) and confocal laser microscope have been used to study the relationship of the resistivity and the etching behavior of the semi-insulating 6H-SiC wafer. Evidence is presented that the morphologies of the etch pits vary significantly with the impurity concentrations. The V impurity strongly affects the etch rates of edge, screw and mixed dislocations. For the dislocation containing the Burgers vector component of <0001>, its vertical etch rate is enhanced distinctly. In contrast, the horizontal etch rate becomes larger for the dislocation containing the Burgers vector component of < >. The shape of the etch pits reflects the Fermi level of the semi-insulating wafer and the net shallow impurity concentration.