Electroflotation (EF) was conducted for activated sludge thickening to investigate the effects of sludge SVI (sludge volume index) and chemical conditioning. Return sludge samples were used for the experiment, which were collected from municipal wastewater treatment plants. The performance of sludge thickening was significantly dependent on sludge SVI. For the sludges with SVI values in a range from 50 to about 150mL/g, the maximum float content decreased rapidly from 8.4 to 3.5% and flotation compressibility followed the same pattern. In cases of sludges with SVI higher than 150mL/g, those results showed low content levels without large changes. Gas/solids ratio tended to increase with an increase in SVI. When polyelectrolyte was added into sludges for the conditioning, compressibility increased up to 75% and gas/solids ratio was reduced up to about 35% under the condition of microbubble production rate of 530mL/h, however, there was no consistent effect of chemical conditioning on the maximum float solids content; some cases were positive but the others negative. It was expected that the optimum dose of electrolyte depends on sludge SVI and an excessive chemical dose causes a performance deterioration of flotation thickening.