Abstract: In order to obtain the size of a focus spot of continuous phase plate (CPP), an edge-abstracting method is proposed. It is possible to acquire a false edge profile if traditional edge-abstracting methods are used because CPP focus spot image has a great grayscale grads. After the operations of grads processing, threshold segmentation, spatial scanning and ellipse fitting, the focus spot edge can be obtained. The edge-abstracting results are compared separately with Laplace, Sobel, Krisch, Prewitt, Log arithmetic operators for grads processing, which shows that the results are more robust and reliable if Sobel or Prewitt arithmetic operators are used. And the abstracted results are compared with different threshold values for threshold segmentation, which shows the results are more reliable if the threshold value is 110 to 170 times of the grads mean value of the background grayscale levels.