The partial cDNA sequences of MHC I genes were obtained from grass carp (Ctenopharyngodon idella) genome sequence based on local Blast against zebrafish's MHC I region. Gene expression levels of BRD2, KNSL2, RXRB, TAPBP, FABGL, MHC Ia, PSMB9 and TAP2 in various tissues were detected by semi-quantitative RT-PCR. The results showed that expression of 7 genes above were highest in the blood but that of FABGL was highest in the kidney. After Aeromonas hydrophila infection at 4, 12, 24, 48 and 96 h, expression level of immune-related MHC I genes was detected in the spleen and kidney with quantitative realtime PCR(qRT-PCR). The expression level of BRD2 gene reached the peak at 12 h in the kidney but at 24 h in the spleen. The expression level of MHC Ia gene increased first and then decreased both in the spleen and kidney, reaching the maximum at 24 h. In the spleen, the expression of PSMB9 increased signicantly and reached the peak at 12 h, and then decreased signicantly at 48 h, following by a recovery to normal level at 96 h; in the kidney, the expression peaked at 24 h, and then continued to decline. The expression levels of TAP2 gene were first down regulated and then up regulated in the spleen and kidney, and backed to normal level at last. It is indicated that MHC I genes are related to the immune of grass carp, laying a foundation for further studying fish diseases and breeding resistance cultivars.