We hanged the activated sludge floc on an elastic nylon stick, and then measured its displacement when the floc was subjected to a uniform water flow. The hydrodynamic drag force exerted on the floc was subsequently estimated, both for cationic flocculated locs, and for the flocculated then freeze/thawed floes. The drag force ranged 0.01-0.06 dynes over Reynolds number of 11-27. These results could be used as the benchmark for floc model development. The three-dimensional model reconstructed from the confocal laser scanning microscope (CLSM) images revealed the complex structure within the floc.