Abstract: The fragments of 18S rRNA gene were sequenced for 7 species  from 7 families of Acridoidea, and the homologous sequences of 17 species of Orthoptera were downloaded from the GenBank data library. After the sequences were aligned, likelihood ratio test was used to find the best model of nucleotide substitution fitting the data obtained from alignment. The molecular phylogenetic trees were reconstructed using Paratettix cucullatus of Tetrigoidea and Stiphra robusta of Eumastacoidea as outgroups with NJ, MP, ML and Bayesian methods. There were 205 variable sites and 74 parsimony-informative sites in the 1 849 nucleotides of the data, and the relative frequencies of the four nucleotides were approximately equal, which showed that the base compositions were not biased. The molecular phylogenetic trees showed that Acridoidea was divided into four clades, and the present families including Chrotogonidae, Pyrgomorphidae, Catantopidae, Arcypteridae, Gomphoceridae and Acrididae, were not monophyletic. It is suggested that Acridoidea be divided into four families:  Pyrgomorphidae, Pamphagidae, Pneumoroidae and Acrididae.

Key words: Orthoptera, Acridoidea, 18S rDNA, molecular phylogenetic tree, phylogeny, taxonomic system