This paper describes the details of the traces that we found at gates of Angkor Thorn, and studies the building phases of gates leading by the traces. B.Ph.Groslier concludes that gates of Angkor Thorn were constructed in three stages. According to him, gates were initially a structure composed from three towers. The decorations were later added at the corners of the tower and finally, towers were scrapped and arranged, and stone materials were added to the top of towers on which faces were carved. We reviewed his three stages, concluded that his opinion is correct. We found the doorway with wooden lintel between the first chamber and the second chamber in all gates. Our investigation points to the possibility that the second chamber was added to the first chamber later on.