Abstract: Xanthomonas campestris pathovar campestris (Xcc) causes serious black rot in all cruciferous plants worldwide. Gram-negative phytopathogenic bacteria including Xcc cause disease in host plants and trigger a HR (hypersensitive response) in non-hosts or resistant host plant through injecting type III secreted effectors (T3SEs) into host cells. T3SEs are a class of proteins secreted by the conserved type III secretion system (T3SS) which is encoded by hrp cluster. To investigate the functionality of hrp genes per se in hrp cluster, the deletion mutants for each hrp gene were constructed. The results showed that each mutation of the nine hrc genes abolished virulence in Chinese radish and lost HR induction in pepper ECW-10R. As for the nine hrp genes, only the knockout of hrpW triggered a tardy HR and decreased virulence, others lost virulence and HR induction completely. The virulence and the ability to trigger HR abolished in hpaA and hpaB mutants, while the deletion of hpa1 and hpaP reduced virulence and HR significantly. Furthermore, transcription analysis through RT-PCR demonstrated that all members of hrp cluster in Xcc 8004 were activated by hrpG and hrpX at different levels.