To understand the evolutionary relationship among six Tetranychus species in Korea, 598 nucleotides of cytochrome oxidase subunit I(COI) were sequenced and analyzed. The sequences were extremely rich in A+T(77% on average). The nucleotide differences in pairwise comparison among species ranged from 6.7% to 14.9%. Nucleotide substitutions based on all pairwise comparison showed 55% of transversions. The largest genetic distance was found between species T.viennensis and T.piercei. The inferred phylogenetic tree indicates that T. urticae, T. cinnabarinus and T.kanzawai are clustered together and showing sister-group relationship with T. truncatus at the highest boostrap support. T. viennensis showed the largest genetic distance from all other species. T.piercei, a species recently found in Korea, was also distinct fro the group composed of four species and also from T.viennensi. The restriction sites of Alu I, Dde I and Sau 3A were searched in the sequences of COI. The restriction fragment length polymorphisms (RFLPs) in COI genes and intraspecific variabilities of three restriction sites in diverse colonies of each species collected from different locations and various host plants were shown.