The effect of cashew nut shell liquid (CNSL) feeding on bacterial and archaeal community of the bovine rumen was investigated by analyzing clone libraries targeting 16S rRNA genes, methyl‐coenzyme reductase A‐encoding genes (mcrA), and their respective transcripts. Rumen samples were collected from three non‐lactating cows fed on a hay and concentrate diet with or without CNSL supplementation. DNA and complementary DNA (cDNA) libraries were generated for investigating rumen microbial communities. MiSeq analysis also was performed to understand more comprehensively the changes in the microbial community structures. Following CNSL supplementation, the number of operational taxonomical unit (OTU) and diversity indices of bacterial and archaeal community were decreased. Bacterial OTUs belonging to Proteobacteria, including Succinivibrio, occurred at a higher frequency with CNSL feeding, especially in cDNA libraries. The methanogenic archaeal community became dominated by Methanomicrobium. A bacterial community shift also was observed in the MiSeq data, indicating that CNSL increased the proportion of Succinivibrio and other genera known to be involved in propionate production. Methanogenic archaeal community shifts to increase Methanoplanus and to decrease Methanobrevibacter also were observed. Together, these results imply the occurrence of significant changes in rumen communities, not only for bacteria but also for methanogens, following CNSL feeding.