This paper conducted Electro-Contact Discharge (ECD) dressing experiment for #600 diamond grinding wheel to understand how fine diamond grits protrude from metal-bonded wheel surface. The SEM observation, EDS analysis, image processing and 3D grit modeling on wheel surface were carried out to investigate grit protrusion characteristics. Then ECD dressing and mechanical dressing experiments were carried out to identify the effect of grit protrusion feature on grinding performance. It is confirmed that the dressed wheel surface topography is sensitive to open circuit voltage Ei, discharge polarity and electrode composition. Meanwhile, ECD dressing with Ei=15V and straight polarity can produce superior protrusion topography without the damage of diamond crystal faces and the bond tail behind protrusive grit. It can obtain better ground surface of hard-brittle material than mechanical dressing with the bond tail.