To understand the greatly enhanced settleability of aerobic granular bio-particles, bacterial alginate, one component of extracellular polysaccharides, was extracted both from aerobic granular bio-particles and the seed activated sludge. Their guluronate to mannuronate radio, reaction with Ca2+, and the gel formation property were investigated and compared by the FT-Raman spectroscopy and atomic force microscopy. The larger amount of guluronate rich alginate (310 +/- 16 mg g(-1)) in the bio-particles inclines to integrate into strong gel, which becomes the structural polymer and promotes the bio-particles formation. By enlarging the particle size, increasing the density and hydrophobicity, bacterial alginate tremendously improves the granular bi-particles' setteability.