Black liquor is a remnant of pulping fluid which generally consists of 14-18 percent solids which is commonly known as weak black liquor (WBL) with a pH of 12 or more. About 65 percent of the solids contained in black liquor are organic compounds derived from wood such as lignin, resins and fatty acids, acids from carbohydrates, and other organic compounds; The remaining 35percent percent is white liquor which is used in the pulping process. Kraft lignin, also called sulfate or alkali lignin, is obtained from black liquor by the extraction method using an acid hydrolysis process, so that pure kraft lignin compounds can be obtained which can be further utilized in various industrial activities. This study is comparing two kraft lignin extraction methods of acid hydrolysis process with thermal assistance (heating temperature) and a gradual centrifuge. The experiment results showed that the levels of lignin contained in black liquor consist of total calories of 391 kcal / Kg which can be processed into energy sources in the recovery boiler unit in further process. The result of kraft lignin extraction process using the acid hydrolysis method with gradual centrifuge stage is better than the thermal acid hydrolysis method. Quantitatively calculated with ASL values difference of around 0.002 mg / g of both the method and the AIR Value using centrifuge give an average yield of 969.815 mg / g, while using thermal assistance gives an average yield of 679.790 mg / g. Qualitatively, the centrifuge method is more optimal compared to the thermal method shown by the results of ICP, FTIR and SEM testing.