Abstract： Objective To make qaulititive and quantitative analysis of Archaea in subgingival plaque sample and to investigate the relationship between periodontal disease and Archaea.Methods Subgingival plaque was collected from 23 patients with aggressive periodontitis,29 with chronic periodontitis,35 with plaque-induced gingivitis and 38 healthy controls.Qualitative and quantitative analysis of methanogenic archaea was performed by amplification of the 16S rRNA genes in the DNA extracted from the plaque samples.Results Archaea were found in 65% of aggressive periodontitis patients,72% of chronic periodontitis,26% of ginginvitis and zero of healthy subjects.Quantitative analysis showed the average abundance of archaeal 16S rRNA gene in Archaea-positive patients was different among the three groups.The average 16S rRNA gene copy number from per μg wet plaque was 6.66×106 in aggressive periodontitis sufferers,4.47×106 in chronic periodontits and 1.78×106 in ginginvitis groups.The prevalence of Archaea and the average Archaea 16S rRNA gone numbers in periodontitis groups were higher than those in gingivitis group(P＜0.05).Conclusions This suggests that Archaea may be implicated as causative agents for periodontits.