This paper proposes an edge detection method by Canny operator with a feedback strategy for glomeruli extraction. As we know, the effect of the Canny operator is determined by three parameters: high threshold, low threshold and standard deviation. To obtain the appropriate parameters for each image, Otsu method is used to set high and low thresholds of Canny operator firstly. And then, to select the optimal standard deviation, a feedback strategy is developed. After parameter selection, Canny operator is applied to our renal biopsy images and experimental results show that some samples can achieve successful extraction and the others result in the discontinuous edges of glomeruli. In the case of the latter, the endpoints in an image should be located and connected to form a whole edge of glomerulus. The experiments have produced the promising results for our samples.