1. A method for image recognition of an instrument, the method to be implemented by a system including an image processing unit and a database, the database storing a plurality of groups of sample images, each of the groups belonging to a respective one of a variety of instrument categories, each of the instrument categories corresponding to a respective priority value, the method comprising the steps of: obtaining, by the image processing unit, an input image containing a to-be-recognized instrument; selecting from the input image, by the image processing unit, a region-of-interest containing the to-be-recognized instrument; determining in a high-to-low order of the priority values of the instrument categories, by the image processing unit, whether the to-be-recognized instrument contained in the region-of-interest belongs to one of the instrument categories according to the region-of-interest and the respective one of the groups; and increasing, by the image processing unit, the priority value corresponding to said one of the instrument categories when it is determined that the to-be-recognized instrument contained in the region-of-interest belongs to said one of the instrument categories.