Rainbow trout (Oncorhynchus mykiss) is a species of cold-water fish with important economic values, which is widely cultivated around the world. It is susceptible to infectious hematopoietic necrosis virus (IHNV), resulting in a large number of deaths. In this study, we measured the changes of immune parameters in different periods (6-, 12-, 24-, 48-, 72-, 96-, 120-, and 144 hours post infection (hpi)), transcriptome profiles of T48G/C48G and important immune-related genes expression patterns in rainbow trout gill following IHNV challenge through biochemical methods, RNA-seq and qRT-PCR. The results showed that AKP, ACP, T-SOD, ALT and AST activities, as well as MDA and LZM content decreased and then increased during infection, and reached to the lowest value at 48 hpi (P < 0.05), whereas CAT activity showed significant peaks at 48 hpi (P < 0.05). The mRNA and miRNA analysis identified 4343 differentially expressed genes (DEGs) and 11 differentially expressed miRNAs (DEMs), and numerous DEGs involved in immune responses, such as tlr3, tlr7, tlr8, traf3, ifih1, trim25, dhx58, ddh58, hsp90a.1, nlrc3, nlrc5, socs3, irf3, irf7, mx1, vig2, bcl2, casp3 and casp8?Integrated analysis identified four key miRNAs (miR-27d-3p, miR-124-5p, miR-301a-5p and miR-146-5p) that target important immune-related DEGs (tlr3, ifih1, vig2, irf8, socs3 and cox1). Gene Ontology and Kyoto Encyclopedia of Genes and Genomes analyses showed that DEGs and target genes were significantly enriched in various immune-related terms including inflammatory response, DNA replication, cell adhesion and pathways of Toll-like receptor signaling, apoptosis, DNA replication, p53 signaling and MAPK signaling. The expression trends of 15 DEGs and 2 DEMs were further verified by quantitative real-time PCR, which confirmed the reliability of the transcriptome sequencing results. Expression analysis showed that tlr3, tlr7, traf3, ifih1, trim25, dhx58, hap90a.1, irf3, irf7 and mx1 genes increased significantly after 48 hpi during infection. This study contributes to the understanding of the immune response of rainbow trout against IHNV infection, and provides new insights into the immune regulation mechanisms for future studies. Overall design: In this study, we measured the changes of immune parameters in different periods (6-, 12-, 24-, 48-, 72-, 96-, 120-, and 144 hours post infection (hpi)), transcriptome profiles of T48G/C48G and important immune-related genes expression patterns in rainbow trout gill following IHNV challenge through biochemical methods, RNA-seq and qRT-PCR.