High surface area activated carbons (ACs) were prepared by chemical activation of black liquor with potassium hydroxide. The influence of carbonization temperature, activation temperature and impregnation ratio of activating agent to char on the porous characteristics of the ACs was investigated. The results show that the surface area and pore volume of ACs, which were estimated by BET methods, were achieved as high as 3089.2 m2 g−1 and 1.7595 cm3 g−1 under the condition of an carbonization temperature of 600°C, an activation temperature of 900°C, and an impregnation ratio of 2. In addition, the influence of alkali metals (sodium and potassium) existing in black liquor on activation was also examined. It was found that the alkali metals react as an activating agent in the process of activation, thereby economizing the amount of activating agent used in this study to a high degree.