Abstract: This paper dealt with the nucleotide composition and the phylogenetic relationship among 8 species in Pentatomidae (Pentatominae,Asopinae，Tessaratominae,Scutellerinae) based on 432 bp sequences of Cyt b gene.The results showed:the average contents of T,C,A and G are 31.3%,12.4%,37.6% and 18.7%,respectively;the contents of A+T (68.9%) is obviously higher than that of C+G (30.1%),and come up to 82.7% at the third position of codon.The divergence of sequences is higher among genera and species,and replacement of nucleotides mostly occurs at the third position.The bug,Megacopta cribraria regarded as outgroup,the phylogenetic tree is set based on molecular information.Altogether with morphology characters,it is very reasonable that the two subfamily,Tessaratominae and Scutellerinae should be separated from family Pentatomidae,and be two valid families.The phylogenetic relationship of them is shown as follow:Tessaratomidae and Scutelleridae constitute a sister group and diverge more early than Pentatonidae;as a monophyletic clade,Pentatomidae is the most evolutionary group in Pentatomoidea.