Three-dimensional integrated circuit (3DIC) structures are disclosed. A 3DIC structure includes a first die and a second die bonded to the first die. The first die includes a first integrated circuit region and a first seal ring region around the first integrated circuit region, and has a first alignment mark within the first integrated circuit region. The second die includes a second integrated circuit region and a second seal ring region around the second integrated circuit region, and has a second alignment mark within the second seal ring region and corresponding to the first alignment mark.