Objective To investigate the inhibitory effect on rat MHC Ⅱ transactivator (C Ⅱ TA) and MHC Ⅱ gene expression by plasmid vector-based RNAi technology. Methods According to C Ⅱ TA ge-netic information, three short hairpin RNA (shRNA) sequences were designed and the corresponding plas- mid vectors were constructed. Rat dendritic cell (DC) was transfected in vitro and rat spleen was transfected in vivo. Real time RT-PCR and flow cytometry were used to detect the expression of C Ⅱ TA and MHC Ⅱ in DC and spleen after transfection. Results C Ⅱ TA-shRNA plasmids were successfully constructed. Com-pared with control groups, the mRNA transcription levels of C Ⅱ TA and MHC Ⅱ and the expression level of MHC H were significantly inhibited in all three pC Ⅱ TA-shRNA experimental groups ( P < 0.01 ). There was positive correlation between the expression of C Ⅱ TA and MHC Ⅱ. Among the three shRNA groups, the first one showed the strongest inhibitory effect. Conclusion The expression of rat C Ⅱ TA and MHC Ⅱ can be obviously inhibited by plasmid vector of shRNA targeting C Ⅱ TA gene, which can be used for further investi-gation of gene therapy.