Although some novel antimicrobial peptides (AMP) have been successfully isolated from Bactrocera dorsalis Hendel, the mechanisms underlying the induction of these peptides are still elusive. The homolog of NF-kappa B transcription factor Relish, designated as BdRelish, was cloned from B. dorsalis. The full length cDNA of BdRelish is 3954 bp with an open reading frame that encodes 1013 amino acids. Similar to Drosophila Relish and the mammalian p100, it is a compound protein containing a conserved Rel homology domain, an IPT (Ig-like, plexins, transcription factors) domain and an I kappa B-like domain (four ankyrin repeats), the nuclear localization signal RKRRR is also detected at the residues 449-453, suggesting that it has homology to Relish and it is a member of the Rel family of transcription activator proteins. Reverse transcription quantitative polymerase chain reaction analysis reveals that BdRelish mRNAs are detected in different quantities from various tissues and the highest transcription level of BdRelish is determined in fat body. The injection challenge of Escherichia coli and Staphylococcus aureas significantly upregulated the expression of BdRelish. The injection of BdRelish dsRNA markedly reduced the expression of BdRelish and decreased the transcription magnitude of antimicrobial peptides. Individuals injected BdRelish dsRNA died at a significantly faster rate compared with the control groups. Therefore, BdRelish is vital for the transcription of AMPs to attack the invading bacteria.