An anti-counterfeit method and system by using a nano metal grating, wherein an object to be recognized is formed with the nano metal grating. During the recognition process, an incidental light is applied on the metal grating. In response to the incidental light, the metal grating has its specific optical incidence and reflection characteristics. The object is then identified whether it is genuine or fake by observing the refraction light passing through and reflection light reflected from the metal grating. Further, the incident light beam can be polarized to generate polarized light beams and then irradiated on the metal grating, whereafter the recognition of the object is performed by observing the reflection and refraction lights of the irradiated polarized light beam. The recognition further can be accomplished by rotating the metal grating, thus the intensity of the refraction and reflection lights will accordingly be changed, thereby identifying the object.