The Type VI Secretion System (T6SS) provides enhanced virulence to Campylobacter jejuni and has been associated with a high incidence of bloody diarrhea. The hemolysin-coregulated protein (Hcp)-the hallmark of the T6SS-can act as a structural and effector protein. Unlike other T6SS-harboring bacteria， which possess multiple Hcp proteins each performing different functions， C. jejuni possesses only one Hcp protein， and its structural and functional role has not been elucidated previously. Here， we report the structure and functional studies of Hcp from C. jejuni. We found similarities between the hexameric ring structure of Hcp-Cj and that of Hcp3 from Pseudomonas aeruginosa. Through functional studies， we showed two roles for Hcp-Cj that is， in cytotoxicity toward HepG2 cells and in biofilm formation in C. jejuni. In structure-based mutational analyses， we showed that an Arg-to-Ala mutation at position 30 within the extended loop results in a significant decrease in cytotoxicity， suggesting a role for this loop in binding to the host cell. However， this mutation does not affect its biofilm formation function. Collectively， this study supports the dual role of Hcp-Cj as a structural and effector protein in C. jejuni.