Anaerobic digestion is an interesting solution to wastewater sludge management issue as a part of organic matter is converted into methane which is valorisable as renewable energy. The aim of this study was to investigate and to compare the impact of sludge thermal pre-treatments on semi-continuous anaerobic digestion of two waste activated sludge samples collected from different urban wastewater plants. Chemical oxygen demand, total solids and organic solids removal rates and methane yields were determined after reactor stabilization. All the studied thermal pre-treatments (130 degrees C, pH = 10, 150 degrees C and 170 degrees C during 30 min) led to an enhancement of anaerobic digestion performance compared to untreated sludge samples. For one sludge, the 130 degrees C, pH = 10 and 170 degrees C treatments led to the same sludge solubilisation and the same anaerobic digestion performance. However, alkali addition had negative effect in total solid removal. For the second sludge, the 170 degrees C treatment led to higher solubilization rates and higher methanization performance than the 150 degrees C treatment. As both sludge samples were quite similar, the same 170 degrees C treatment led to comparable results in anaerobic digestion performance increase: about 80% improvement in removal of matter and in biogas yield. Nevertheless, values of methane yields were slightly higher for the most organic sludge.