The complete gene sequence of the outer membrane protein W (OmpW) was amplified from the genomic DNA of Aeromonas hydrophila Wp3 strain which was isolated from grass carp suffered hemorrhage. The OmpW gene was 865 bp in length, containing an open reading frame (ORF) of 615 bp. The cloned gene possessed high similarity with that of the standard strain ATCC7966 (99.8%). The sequence encoding the mature peptide of OmpW was amplified and inserted into the expression vector pQE30. The recombinant vector was transformed into E. coli, and a 24.7 kD recombinant fusion protein His-W was expressed. Elisa analysis of the serum of grass carp immunized with His-W showed a positive immune reaction, suggesting production of the antibodies. Total RNA of the head kidney of the immunized grass carp was extracted and mRNA level of IgM was analyzed by qRT-PCR. Expression levels of IgM gene in the immunized group were higher than those of the control group, and significant difference (P<0.05) was found in the lowest protein concentration group (2 μg/g). These results showed that His-W was able to efficiently induce the immunized grass carp to produce antibodies. Protection experiments showed that the immunized groups had higher survival rates than those of the control groups (57%?86%). This study suggested that the recombinant protein His-W was a candidate vaccine for grass carp A. hydrophila disease.