A method to determine whether a pixel is the edge point of a color image is presented. First, according to the characteristic of RGB color space, a color triangle of a pixel is constructed with its color coordinates RGB, and then the perimeter and the internal angle of the color triangle which can be seen as the information value of a pixel is calculated. Last, comparing the value with that in its neighborhood, a pixel is in the edge point or not is determined. This method took the correlation of every color component into account reasonably, and converted the computation of a vector space to scalar quantity in a natural way. Comparing it with traditional method, experimental result shows that more legible edges can be detected with this method and can be realized easily.