It is known that French forces were repeatedly defeated by the so-called "Black Flags" led by Liu Yung-fu during Sino-French war (1884～5). This article treats of Tu-Duc's policy to control bandits, which made possible to connect Liu Yung-fu with Nguyen and Ching governments. Liu Yung-fu was originally the leader of a branch of riots having been active along the border of Kwangsi during Taiping-Revolution and driven to Viet-nam by pressure of governmental troops of Ching Dynasty; he was also an intruder, a common enemy both for China and for Viet-nam. He, however, was obliged to be confronted by French forces when they reached Tongking after having conqured Cochinchina. Moreover he was threatened by Jean, Dupuis, French marchant, who made use of the Red River as a navigable route to Yunnan. Because Liu Yung-fu's main base…Lao-Kai…situated in the basin of the River. The fact that his hostile rival "Yellow-Flags" allied themselves with French forces drove him rapidly to join Nguyen Dynasty. On the other hand, Tu Due assumed a conciliatory attitude to these bandits after he had failed in driving them away from his dominion. He wanted to let them fight against the French, giving them official ranks, permitting to settle down in his land, offering money and grains, admitting them to charge customs and so forth. Thus Liu Yung-fu and Nguyen authorities co-operated togather for their common interest. Huang Tso-yen, the mediator between Tu Due and Liu Yung-fu, was originally the responsible commander for suppressing bandits. He utilized the "black-Flags" to control other bandits and French forces. The relation between Liu Yung-fu and Huang Tso-yen gradually grew worse, becaue of latters discontent with rewards. Finally Liu Yung-fu disregarded the orders from Huang Tso-yen, but he had no confidence in standing on his own feet. When China began to consider the defence against French influence over Sino-Vietnamese border and dispatched Tang Ching-sung, Lui Yung-fu found a chance to ally himself with Ching Dynasty. Following sugestion of Tang Ching-sung, he declared his intention to resist French forces. The main reason why Liu Yung-fu led the attack on French forces was that he desired to maintain his base at Lao-Kai. In conclusion, Tu Due's conciliatory policy only resulted in preserving Liu Yung-fu's power. It will be a mistake to regard him as a true nationalist and a great contributor to the establishment of the relationship between China and Viet-nam.