A knife sharpener may comprise a main body, and a suction cup formed on a lower surface thereof is operated by an operating handle. By cooperating the main body with the operating handle and the suction cup, the knife sharpener is configured to easily adhere to a flat surface, which avoids a damage formed on the flat surface during the sharpening process. Moreover, since the knife sharpener comprises various sharpening units, it can be used to sharpen different kinds of knives, even a scissors comprising a pair of blades, which greatly enhances the practicability thereof.