Pretreatment of thickened waste activated sludge (TWAS) can facilitate the hydrolysis step of mesophilic anaerobic digestion (MAD) and improves the digestibility. This study examined the effects of microwave/alkali pretreatment on TWAS disintegration and its subsequent batch and semi-continuous MAD. The maximum solubilization ratio of volatile suspended solids, 77%, achieved at 210°C with 0.15 g-NaOH/g-TSS and 10 min holding time. In the biochemical methane potential tests, the vials fed with pretreated TWAS produced 18-46% higher biogas compared to the control. The semi-continuous reacter fed with the pretreated TWAS without neutralization was stable, and the VS reductions and methane yield were 21% and 14% higher than those of the control, respectively.