An image sensor device includes a substrate, a photo sensitive element, a first dielectric structure and a convex dielectric lens. The substrate has a first side and a second side opposite to the first side. The photo sensitive element is formed on the first side of the substrate for receiving incident light transmitted through the substrate. The first dielectric structure is formed on the second side of the substrate. At least one portion of the convex dielectric lens is located in the first dielectric structure. The convex dielectric lens has a convex side oriented toward the incident light and a planar side oriented toward the photo sensitive element.