This paper makes clear characteristic of the urban space under the early modern times of the county capital of Jiadin (嘉定) which is located in the region of canals (or rivers and lakes) of Jiangnan (江南) in China. Mainly on the influence of the theory of Fengshui (風水). And so it gains consideration based on the literal document and the practical investigation result of the place. The main structure of the county capital of Jiadin was built in 1217. I think that the city wall, the main urban facilities and so on might be completed approximately in the late Ming period and they have been kept without changing since then. I also think that the tide of the canals which is seen in Jiadin outer place of the Gugangshen (古岡身) in the Changjiang(長江) delta area, might influence urban space. About the urban study of Jiangnan, there are many studies on Suzhou (蘇州), the main big city in the area and on market towns in the outskirts of Suzhou. However, as for the county capital, which existed between them in size, though it is the core of politics, culture and economy in the area, it has not collected attention. There was an only approach by Dr.Yonekura in 1987 on the point of village geography. As for the composition of the urban space, I have ever made a couple of paper. As for the influence of the theory of Fengshui, a study report on the cities which seat in the mountain zone is seen, but in the region of canals of Jiangnan there has been no study yet. The built-up area at the county capital of Jiadin may have composed the unique urban space depending on the traffic on water as the port town although having agreed by circumvallation. As for it, I could point out that, the built-up area is formed only along the waterway and on the intersection point of the waterway or at a port town attached to the city near a castle gate, was the most prosperous. Also, Considering like this way, The urban space at the county capital of Jiadin has the character of the politics city = the castle city, whereas there a