Objective To develop directly molecular evolution of nitrite oxido-reductase using DNA-shuffling technique because nitrobacteria grow extremely slow and are unable to nitrify effectively inorganic nitrogen in wastewater treatment. Methods The norB gene coding the nitrite oxido-reductase in nitrobacteria was cloned and sequenced. Then,directed molecular evolution of nitrite oxido-reductase was developed by DNA-shuffling of 15 norB genes from different nitrobacteria. Results After DNA-shuffling with sexual PCR and staggered extension process PCR,the sequence was different from its parental DNA fragments and the homology ranged from 98% to 99%. The maximum nitrification rate of the modified bacterium of X16 by DNA-shuffling was up to 42.9 mg/L·d,which was almost 10 times higher than that of its parental bacteria. Furthermore,the modified bacterium had the same characteristics of its parental bacteria of E. coli and could grow rapidly in normal cultures. Conclusion DNA-shuffling was successfully used to engineer E. coli,which had norB gene and could degrade inorganic nitrogen effectively. Objective To develop directly molecular evolution of nitrite oxido-reductase using DNA-shuffling technique because nitrobacteria grow extremely slow and are unable to nitrify effectively inorganic nitrogen in wastewater treatment. Methods The norB gene coding the nitrite oxido-reductase in nitrobacteria was cloned and sequenced. Then, directed molecular evolution of nitrite oxido-reductase was developed by DNA-shuffling of 15 norB genes from different nitrobacteria. Results After DNA-shuffling with sexual PCR and staggered extension process PCR, the sequence was different from its parental DNA fragments and the homology ranged from 98% to 99%. The maximum nitrification rate of the modified bacterium of X16 by DNA-shuffling was up to 42.9 mg / L · d, which was almost 10 times higher than that of its parental bacteria. had the same characteristics of its parental bacteria of E. coli and could grow rapidly rapidly in normal cultures. Conclusion DNA-shuffling was succe ssfully used to engineer E. coli, which had norB gene and could degrade inorganic nitrogen effectively.