From the 10-year research in graphic maps of Thang Long citadel before the 19th century, chiefly Thang Long citadel from the Le period in the late 15th century - late 18th century, the author realizes that these maps are similar to each other in that they all include Thang Long citadel in building ruler shape before the 19th century, the central North-South axis from Van Tho palace (Kinh Thien) through Doan Mon to the Flag Pole and the North-West corner of the citadel in Buoi precinct (Tay Ho district). The differences in this map system are the locations of the palaces in the Imperial citadel and the diversion out of East-West boundary in the North and south citadel. However, from this map system, it is possible to identify the location and perimeter of Thang Long citadel before the 19th century as follow: 1. The North-South axis in the middle of the Imperial citadel is at least as wide as the East-west Quoc Tu Giam area or the distance of about 200m between Chu Van An and Hoang Dieu streets. The north citadel wall coincides Phan Dinh Phung street, the south citadel wall is in the same location of Nguyen Thai Hoc street. 2. The East-West width of the Imperial citadel is larger than that of Hanoi citadel built in 1802. The North-South axis of Hanoi citadel with the Flag Pole as a standard landmark is not in the same location of the central axis of Thang Long citadel before the 19th century. If the East wall of the Imperial citadel is considered to coincide the East Hanoi citadel and the North-South central axis from Quoc Tu Giam (south) to Tran Vo Quan (north), the west Thang Long citadel is equal to the east side. 3. It is possible to identify that Thang Long citadel perimeter includes the entire interior land (the 19th century), the North side runs along To Lich River, the length is from Phan Dinh Phung - Hoang Hoa Tham to Buoi market. Along To Lich river towards Cau giay street through Giang Vo and Kim Ma, along Nguyen Thai Hoc street is the south wall and the east wall coincides the east Imperial citadel.