This study explores the establishment and competency change of the Patrol Division of Le Dynasty. Patrol Division was established in Le-so （1428-1527） and Originated in the Ming Dynasty. It was mostly founded at water channels and coastal river mouths of different places. The original function was to inspect Chinese spies along the coast and water channels. It was the same as the Patrol Division established at “channels of pass and ferry” in the Ming Dynasty，and its functions are also very similar.

Afterwards，the competency of the Patrol Division changed in Le-Trung-Hung （1533-1789）. At the time，for the concern of powers of Ming Cheng in Taiwan，the Qing court implemented the prohibition on maritime trade or intercourse with foreign countries. Cheng in Taiwan cooperated with other maritime powers，including the pirate groups of Deng （Yao） Yang （Yan Di） in the Gulf of Tonkin. Besides，it practiced various commercial trades with western countries on the ocean of East Asia，including smuggling or piracy. Vietnam was certainly part of the maritime trades. Water channels of Patrol Division were the important ones of commercial trades and acquisition of goods. According to the monopoly law of copper and Chinese cinnamon established in the first year of Bao-Tha （保泰，1720），the businessmen mostly entered Bac-Ky（北圻） from the sea route for trading. They purchased a great amount of copper and Chinese cinnamon. Commercial trading violated the regulation which forbade the businessmen to trade in Bac-Ky. Since Patrol Division traveled around the water channels，they approached and extorted businessmen，which resulted in the change of competency of the Patrol Division.

Thus，the establishment of the Patrol Division was originated from the political relation with China，and afterwards，competency was changed due to the impact of maritime trading. It showed the vigorous maritime trading between northern Vietnam and China. The demand for copper and cinnamon significantly influenced Patrol Division. Hence，the monitoring competency of the Patrol Division was changed to the collection of business taxes in the middle and late periods of Le-Trung-Hung.