Abstract: In this study,the genetic relationships among five species belonging to four genera of Mermithidae were detected using RAPD. Twelve primers were screened from 47 random primers and amplified 161 bands (from 200 bp to 3 200 bp). 150 (93.17%) of these bands were polymorphic bands. The number of obtaining segments of individual primer was between 11-16,the average was 13.42. Neis similarity coefficients and genetic distance were calculated using the RAPDistance and MEGA softwares. The phylogenetic trees were constructed with the methods of UPGMA and NJ. The results from two methods of cluster analysis were similar in general,and the relationship indicated the five species of Mermithidae could be distinguished with RAPD markers and were grouped into two distinct clusters. In the phylogenetic trees,Romanomermis culicivorax and R. wuchangensis (all parasite in larvae of mosquitoes) grouped together,then they grouped with Agamermis changshaensis (parasite in Homoptera) into one cluster. Ovomermis sinensis (parasite in Lepidopters) grouped with Amphimermis sp. (parasite in Homoptera) into one cluster. The genetic distances of five Mermithidae species showed that the relationships of intra-genus were nearer than that of inter-genus,the genetic distance between R. culicivorax and R. wuchangensis was 0.1789,but the genetic distance between inter-genus were 0.4471-0.5488. All those results suggested that RAPD was suitable for analysis of phylogenetic relationship among species of Mermithidae.