In order to contribute to the accurate reproduction of Thang Long royal citadel planning under the dynasties of Ly-Tran, the author conducts research on the names, locations (layout), and functions of the structures between Thang Long royal citadel with the capital cities of Lac Duong (Luoyang) and Khai Phong (Kaifeng) in China. The results show that the planning and architecture of Thang Long royal citadel in the first phase (1010-1029) were evidently influenced by the models of the capital cities of Llac Duong and Khai Phong, but from the mid-11th century on, those influences were gradually faded due to the strong growth of Dai Viet. Although documentation is very limited at present, the comparative study, more or less, gives us some knowledge and a clearer picture of the space of Thang Long royal citadel planning under the dynasties of Ly- Tran. In addition, the article also provides us some clues to help decipher architectural traces of Thang Long royal citadel unearthed in the area of 18 Hoang Dieu Street (Ha Noi) and help to shape excavation activities in the future.