The present disclosure provides an optical structure and a method for fabricating an optical structure, the method includes forming a light detection region in a substrate, forming an isolation structure at surrounding the light detection region, and forming a primary grid over the isolation structure, including forming a metal layer over the isolation structure, forming a first dielectric layer over the metal layer, and partially removing the metal layer and the first dielectric layer with a first mask by patterning, and forming a secondary grid at least partially surrounded by the primary grid laterally.