The effect of BOD loading on solid-liquid separation properties of activated sludge in wastewater treatment process was investigated. The initial sedimentation velocity of activated sludge is highly dependent on BOD loading, and SVI decreased with increase of BOD loading and became smallest at the BOD loading of 0.4 kg/(MLSS-kg.d). In microfiltration of activated sludge, the dynamic behaviors were well described by the Ruth filtration rate equation. The average specific filtration resistance of filter cake became smallest at the BOD loading of 0.4 kg/(MLSS-kg.d), which is similar to the result of SVI. The filter cake composed of activated sludge has extremely high compressibility, indicating that increase in the filtration pressure does not necessarily lead to increase in the filtration rate. From the experimental data, it was found that the sedimentation and filtration properties of activated sludge correlate with the area mean diameter of sludge flocs. In addition, floc size of activated sludge was greatly influenced by BOD loading of the wastewater. Therefore, it seems reasonable to conclude that the solid-liquid separation properties of activated sludge are closely related to BOD loading of wastewater.