In order to identify virulence factors of the pathogen, the outer membrane proteins of virulent and avirulent strains of Riemerella anatipestifer were compared by a proteome analysis. Three protein spots differentially expressed between the two strains were observed by 2-DE gels, and were further analyzed using in gel tryptic digestion and peptide mass fingerprinting. Three proteins were identified. W1 was Hsp20, W2 and W3 were transposon. Although the exact role of these proteins has not been characterized, the exclusive expression in virulent strain may indicate that they play an important role in the pathogenesis of Riemerella anatipestifer infection. Although only two virulence factors identified, it opens a path to the further analysis of virulence factors of Riemerella anatipestifer.