Three-dimensional integrated circuit structures and methods of forming the same are disclosed. One of the three-dimensional integrated circuit structures includes a first die, a plurality of second dies and a dielectric structure. The second dies are bonded to the first die. The dielectric structure is disposed between the second dies. The dielectric structure includes a first dielectric layer and a second dielectric layer. The first dielectric layer has a sidewall and a bottom, a first surface of the sidewall and a first surface of the bottom are in contact with the second dielectric layer and form a first angle. A second angle smaller than the first angle is formed by a second surface of the sidewall and a second surface of the bottom.