An image sensor includes a sensor unit, a sensing portion disposed within the sensor unit, and an isolation structure corresponding to the sensing portion. The isolation structure includes a first deep trench isolation (DTI) structure surrounding the sensor unit from top view, and a second deep trench isolation structure laterally enclosed by the first deep trench isolation structure. The second deep trench isolation structure is located close to a corner of the sensor unit defined by the first deep trench isolation structure. The second deep trench isolation structure is asymmetrical with respect to a horizontal middle line or a vertical middle line within the sensor unit.