Abstract:Jaoa is an endemic group of fresh water algae in China. The phylogenetic relationship between Jaoa and related algae by using 18S rDNA sequences was studied. The result showed that the length of 18S rDNA of J. prasina is 1632 bp and GC% is 50.6%, and those of J. bullata are 1639 bp and 50.3%, respectively. The maximum parsimonious tree indicated that Jaoa with its related Orders could be divided four Clades. Jaoa prasina, J. bullata, Enteromorpha investinalis, Ulva curvata, U. rigida and Monostroma grevillei constituted strong supported clade (Clade B), among which the lowest nucleotide divergence was 0.041, while that reached up 0.112 between J.prasina, J. bullata and Chaetophora incrassata. Therefore, Jaoa probably belongs to Ulvales. Jaoa prasina and J. bullata constitute a monophyletic branch (100% bootstrap value) in Clade B with divergence of 0.002. It suggested that the two species of Jaoa are closely related species, they might be one species and one variety.