The bioreactor landfill is a solid waste disposal method that provides rapid degradation of solid waste and acquisition of methane. Bioreactors in which leachate circulation is carried out are generally operated anaerobically. The biodegradation of wastes with high lignin and cellulose content is very difficult. Especially under anaerobic conditions (moreover, if there is a lack of moisture), such wastes almost never decompose. In this study, the degradation of waste sunflower stalks that are difficult to biodegrade and have a high lignin-cellulose con-tent and the production of methane gas in semi-aerobic bioreactors have been investigated. Sunflower stalks were loaded into the bioreactors in different proportions and mixed with the organic fraction of municipal solid waste (OFSWM). The bioreactors have been operated un-der different operating conditions. The contents of cellulose, hemicellulose, lignin, and initial and final organic matter in the wastes loaded into the bioreactors were examined. Parameters such as pH, COD, BOD5, TKN, NH4-N in leachate were analysed and the amounts of total and methane gas were measured. Initially, all bioreactors have been operated anaerobically. In the decomposition of the sunflower stalk, while 43% of the organic matter removal was achieved in the anaerobic bioreactor, 60% of the organic matter removal was realized in the semi-aerobic/anaerobic bioreactor. The other agricultural wastes were then subjected to de-composition under semi-aerobic/anaerobic operating conditions. As a result of the study, it can be said that semi-aerobic pretreatment accelerates the decomposition of agricultural waste with a high lignin and cellulose content, decreases the COD values of leachate, and increases the amount of methane.