Abstract: To solve some problems on image enhancement, such as losing details and falling into a sub or over enhancement, a method of image enhancement based on Grümwald-Letnikov fractional directional differentiation algorithm is proposed. According to the basic fractional differentiation form and auto-correlation of digital image, Grümwald-Letnikov fractional differentiation and directional derivative are combined. The method is used to define the coefficients of differentiation enhancement template. Then, a fractional differential convolution template is constructed in every direction. Experimental results show that the proposed method can greatly increase high frequency, effectively improve the low frequency, strengthen the textural detail and edge information of image, and improve the definition and comentropy of image quality index. The enhanced images have better visual quality.