The complete sequence of the 18S ribosomal RNA gene (18S rRNA) from Lymantria dispar was cloned and analysed here. 18S rRNA and mitochondrial cytochrome c oxidasel (cox1) gene sequences of Lymantria dispar were compared with homologous sequences of other nine insects from different orders. Analytic results showed that 18S rRNA of these insects had two conserved domains and the second domain was an even more conserved region. The phylogenetic trees based on the full-length sequence and the second domain fragment of 18S rRNA as well as sequence of cox1 from different orders indicated that Lepidoptera and Trichoptera, which belongs to Amphiesmenoptera, had a closer phylogenetic relationship and fewer differences were observed comparing with traditional taxonomic results.