Abstract: A technique, known as random amplified polymorphic DNA, has been used in this report to analyze DNA polymorphisms in four species of Decapod.The species were Procambarus clarkii, Sinopotamon lansii, Eriocheir japonicus hepuensis and Eriocheir sinensis, which have different pairwise distance.Amplification of DNA pools from four species revealed the similarities among them decreased from 56.1% to 22.3%, as ancestral relationships became more distant,indicating that genome variation increased.The dendrogram based on a UPGMA analysis showed the gradual increase of genetic variation from inter-species to above-family, which was identical to the classic systematics.While little genetic variation between E.sinensis and E japonicus hepuensis was found by allozyme electrophoresis, RAPD revealed certain genetic variation between them.Amplification of DNA from individuals of E.sinensis and E japonicus hepuensis displayed the average similarities were 56.9% in E.sinensis, 66.1% in E.japonicus hepuensis, and 51.9% between two kinds of mitten crabs.The average similarity was significantly higher in E.japonicus hepuensis than in E.Sinensis, and that between two kinds of mitten crabs was significantly lower than in E.sinensis or in E.japonicus hepuensis.