Abstract: Extracting contour(iso-surface) from 3D data field is an important visualization techniques. Marching Cubes algorithm is a method to extract contours. But there is ambiguity in this method. Some methods have developed to solve ambiguity problem,but there are still some unresolved problems. This paper specifies some unresolved ambiguity and provide the method for deciding them and Marching Cubes algorithm that is improved.