Abstract: A novel RbohB gene was isolated from Brassica campestris L. Longyou 6 by RACE. The full-length cDNA of RbohB is 2 694 bp, with open reading frame (ORF) 2 541 bp, encoding a 846 amino acid protein. By RT-PCR analysis, transcript level of RbohB was increased in response to cold, high salt and H₂O₂ stress. However, MAPKK inhibitor U0126 pretreatment 12 h before cold, high salt and H₂O₂ stress, compared with the single stress, the expression level of RbohB was significantly decreasing. The results suggested that RbohB played an important role during cold, high salt and H₂O₂ stress in B.campestris L. and U0126 has inhibitory effect to RbohB gene transcription. The activity level of NADPH oxidase was increased in response to H₂O₂ stress. However, U0126 pretreatment 12 h before H₂O₂ stress, compared with the single H₂O₂ stress, NADPH oxidase activity was significantly decreasing. The result showed that MAPKK inhibitor U0126 has inhibitory effect to NADPH oxidase activity.