Wild type (WT) DNA sequence, which encoded a mature beta-mannanase of Aspergillus sulphureus, composed of 1,152 nucleotides (nt), was amplified from pUCm-T-mann by polymerase chain reaction (PCR). Based on this DNA fragment, mutants designated as E-206 G and E(314)G were constructed by overextension PCR (OE-PCR). Glutamic acids of the 206th and 314th sites in the amino acid sequence of beta-mannanase were separately replaced by glycine in these two mutants. The WT and mutant genes were ligated into prokaryotic vector pET-28a (+) and transformed into the Escherichia coli BL21 strain, respectively. The recombinant enzyme proteins were expressed by IPTG induction and detected by Western blot. The recombinant proteins purified with Ni-NTA column were dialyzed to correctly refold. The WT recombinant beta-mannanase showed optimal activity at 50C and pH 2.4. The kinetic parameters of K (m) and V (max) for purified beta-mannanase were 1.38 mg/ml and 72.99 U/mg, respectively. However, the mutant proteins did not show any activity. It was demonstrated that E-206 and E-314 were the catalytic residues of beta-mannanase.