A β-mannanase-producing bacterial strain Bacillus circulans M-21 was isolated from soil. The strain grew well when mannan (konjac gum, guar gum or locust bean gum) was used as sole carbon source. The optimum fermentation conditions were determined as follows: 4 g/L guar gum as carbon source, 20 g/L soybean powder and 5 g/L (NH4)2HPO4 as nitrogen source, initial pH 8.0, and 32C. An extracellular β-mannanase was purified by anion-exchange chromatography on Q-Sepharose Fast Flow. The purified protein exhibited a single band on sodium dodecyl sulphate-polyacrylamide gel electrophoresis with a molecular mass of 33.4 kDa. Kinetic analysis showed that the enzyme exhibited the strongest binding affinity to konjac gum. The β-mannanase activity reached the maximum at 50C and pH 7.0, and was completely suppressed by Hg2+ and Ag+. The β-mannanase degrading products were mainly composed by disaccharide, trisaccharide and tetrasaccharide, which were all neutral hexose polymers.