A hybrid segmentation algorithm is proposed for automatic segmentation of blood cell images based on adaptive multi-scale thresholding and seeded region growing techniques. Firstly, an adaptive and scale space filter (ASSF) is applied to image histogram and a scale space image is built. According to the properties of the scale space image, proper thresholds can be obtained to separate the nucleus from the original image and the white blood cells are located. Secondly, the local color similarity and global morphological criteria constrain seeded region growing in order to finish the segmentation of the cytoplasm. The detection accuracy of white blood cell is 98% and the segmentation accuracy based on the subjective evaluation is 93%. Test shows that this algorithm is effective for automatic segmentation of white blood cells.