High-solid anaerobic digestions of sewage sludge were investigated in completely stirred tank reactors (CSTR) at 35±1°C. The total solid (TS) concentrations of the sludge fed to the reactors were designed at 10%, 15% and 20% respectively after start-up periods. By start-up period is meant the TS concentration of the substrate in the reactor has not reached its designed level. Special attention was paid to the effect of organic loading rate (OLR), TS concentration, and ammonia inhibition on methane yield, volatile solid (VS) reduction and volatile fatty acid (VFA) concentration during the start-up periods. Experimental results showed that no evident inhibition of TS or ammonia was found as long as the concentration of TS was below 10% or the concentration of ammonia-N was below 2000 mg/l. Digestion at TS concentration of 12.5%-14.8% was slightly influenced by ammonia at concentration of 3200-3800 mg N/l. As the TS concentration was above 14.8%, the ammonia-N concentration of 3500-3800 mg/l showed evident inhibition on methanogetic activity, leading to sharp decrease of methane yield and methane content. After start-up, the methane yields at feeding TS of 10%, 15%, and 20% at OLR of 3.0 kg VS m-3d-3 were 0.221, 0.248 and 0.177 l CH4 g-1VSadded-1 d-1, and the VS reduction were 33%, 39.5% and 40%, respectively.