An optical isolation structure and a method for fabricating the same are provided. The optical isolation structure includes a first dielectric layer, a second dielectric layer, a third dielectric layer and a dielectric post. The first dielectric layer includes a trench portion located in a trench of the semiconductor substrate. The second dielectric layer includes a trench portion covering the trench portion of the first dielectric layer and located in the trench of the semiconductor substrate. The third dielectric layer includes a trench portion covering the trench portion of the second dielectric layer and located in the trench of the semiconductor substrate. The dielectric post is disposed in the trench of the semiconductor substrate and covering the trench portion of the third dielectric layer.