cultivated six-rowed naked barley (hordeum vulgare ssp. hexastichon var. nudum hsü) is the oldest cultivated barley in china. we used 35 simple sequence repeat (ssr) markers selected from seven barley linkage groups to study the genetic diversity, geographical differentiation and evolutionary relationships among 65 h. vulgare ssp. hexastichon landrace accessions collected from the qinghai-tibet plateau of china, 25 accessions from tibet (tb), 20 from qinghai (qh) and 20 from ganzi (gz) prefecture in sichuan province. at the 35 ssr loci we identified 248 alleles among the 65 accessions, 119 (47.98%) of the alleles being common alleles. we also found that the tb accessions possessed 47 private alleles, about 1.5 times more than the 31 private alleles found in the qh accessions and about 5 times more than 9 private alleles found in the gz accessions. generally, the tb accessions showed significantly higher genetic diversity than either the qh or gz accessions whereas no significant difference in genetic diversity was found between the qh and gz accessions. partitioning analysis of genetic diversity showed that about 81% of the total variation was due to within-subgroup diversity and about 19% was clearly accounted for by geographical differentiation among the three subgroups. the distributions of alleles for most loci (71.4%) were significantly different among the three subgroups and geographical differentiation could be found according to the distribution of ssr alleles. cluster analysis indicated that most of the accessions could be clustered into groups which basically coincided with their geographical distribution. these results suggest that tibet might be a center of genetic diversity for cultivated barley, the cultivated six-rowed naked barley on the qinghai-tibet plateau of china may have evolved in tibet and spread to qinghai and then to ganzi prefecture of sichuan province.