Sludge flotation was reported to cause several operational problems in anaerobic systems including UASB reactors treating both strong domestic sewage and some industrial wastewater. This research is to investigate the effect of anaerobic digestion on scum-forming potential (SFP) of sludge and other physical-chemical properties. A simple test was developed to measure and compare the tendency of different sludge to form a scum layer. Results showed that anaerobic digestion affects chemical composition of sludge flocs and consequently, SFP, which was found to be inversely proportional to the degree of digestion (both SRT and temperature). It was suggested that higher protein concentration at elevated SRT and 25 degrees C increased the negative surface charge of sludge flocs and ,consequently, reduced the ability of sludge to attach to gas bubbles and float. Floc average size increased with increasing SRT and temperature, especially for sludge with 75d SRT at 25 degrees C. On the other hand, settling properties of sludge were negatively affected by increasing SRT to 75d at 25 degrees C. Filterability had a strong positive correlation with average floc size, but also polymeric constituents correlated positively with filterability at 25 degrees C.