Random amplified polymorphic DNA markers were used to analyse the DNA diversity of Monostroma nitidum, M. latissimum and M. angicava. The size of genomic DNA of each species was about 23 kb. 43 primers screened from 88 random primers generated repeatedly clear polyotripic bands in all samples, and the proportion was 48.86%. The polymorphic sites proportion of M. nitidum, M. latissimum and M. angicava were 53.14%, (65.28%) and 55.93% respectively. The inter-species genetic distances among those species (M. nitidum, M. latissimum and M. angicava) were 0.411 8, 0.414 8 and 0.437 5 respectively. The result showed that these samples coming from three different marine area were the same genus but different species. Those were consistent with the conventional systemic identification. The molecular phylogenetic tree constructed by UPGMA method coincided with that constructed by NJ method. The result also indicated that the genetic relationship between M. latissimum and M. nitidum was the nearest, that between M. latissimum and M. angicava was farther, and that between M. nitidum and M. angicava was the farthest. The result also testified the fact that M. nitidum and M. latissimum shared more common characteristics in morphology, while M. angicava shared less common characteristics in morphology with M. nitidum and M. latissimum.