Quorum sensing is a control of bacterial gene expression in response to cell density. Several processes in pathogenesis of many medically-important bacteria are associated with quorum sensing. As quorum sensing controls bacterial virulence, the inhibition of quorum sensing is being considered as a novel strategy for antibacterial drug. This study investigated the effect of 44 Thai medicinal plant species on bacterial quorum sensing. Anti-quorum sensing activity was indicated by the inhibition of violacein pigment producing of biomonitor strain, Chromobacterium violaceum DMST 21761. Screening test by disc diffusion assay revealed that 9 plant extracts exhibited anti-quorum sensing activity including Cinnamomum bejolghota, Cinnamomum porrectum, Holarrhena antidysenterica, Punica granatum, Quercus infectoria, Quisqualis indica, Terminalia bellirica, Terminalia chebula, and Terminalia sp. Flask incubation assay was used for quantifying the inhibitory activity. All extracts, except Q. infectoria, reduced violacein production but did not suppress the cell growth. Holarrhena antidysenterica, T. bellirica, and Terminalia sp. produced pronounced inhibitory activity. This finding may uncover the potential mode of action of these plants. Anti-quorum sensing property of these medicinal plants may play an important role as the antimicrobial activity for their efficacy in traditional use as medicines and this should not be overlooked.