Abstract: Cell motility plays a critical role in bacterial chemotaxis, adhesion, biofilm formation, and pathogenesis. There are three major types of bacterial cell motility: swimming, swarming and twitching. It has been shown that swimming and swarming are mediated by flagella, and twitching is mediated by type Ⅳ pilus. Whole genome sequencing revealed that Xanthomonas campestris pv. campestris（Xcc） encodes a complete set of type Ⅳ pilus genes, and Xcc indeed can produce type Ⅳ pilus. However, it has been demonstrated that Xcc can not twitching. Our lab previously found that the type Ⅳ pilus gene pilH and pilG is related to swimming and swarming of Xcc. To determine the biological function of the type Ⅳ pilus gene pilY, in this study, the deletion mutant and its complemented strain were constructed, and then the phenotype of these strain were analysed. Results showed that the swarming motility of the pilY deletion mutant is significantly stronger than that of the wild type strain, while the swarming motility of the complemented strain is identical to that of the wild type strain, demonstrated that pilY functioning as an inhibitor of Xcc swarming motility. In addition, this research found that pilY deletion had no effect on the growth, pathogenicity and production of pathogenic factors of Xcc.