The article presented the circumstances and activities of Au Co guerrilla in Van - Hien Luong war zone in the stage of mobilization for the August revolution. Through historical documents taken from many different sources, the article highlighted the positive work of the guerrilla. Thanks to their activities in this period, Au Co guerrilla quickly became bigger and played an important role in the insurrection to gain the local government. From the research results, some comments on Au Co guerrilla were inferred as follows: 1- Au Co guerrilla and political forces of the masses made enormous strength which could promote the local revolutionary movement to develop quickly. 2- Au Co guerrilla was the main decisive force in the protection of Van - Hien Luong war zone. 3- Au Co guerrilla had created conditions and supports for the masses to seize political power in two provinces of Phu Tho and Yen Bai. This army also continued keeping a very important role in the war against the French colonialism.