The genetic diversity and population genetic structure of Paraoncidium reevesii were investigated using mitochondrial cytochrome c oxidase subunit I(COⅠ)gene sequences.A total of 171 individuals representing 8 collection sites were included in the analysis.Overall,101 haplotypes were dened and 117 polymorphic sites were observed.The P.reevesii populations had high haplotype diversity(0.974±0.005)and nucleotide diversity(0.032 2±0.001 3).The AMOVA test of P.reevesii populations based on haplotype frequencies revealed that 49.01% of the genetic variation occurred within the populations,whereas 50.99% of the genetic variation occurred among populations.Pairwise xation index(F_st),gene flow(N_m)and IBD analysis indicated signicant genetic structure have appeared in P.reevesii populations.Demographic analyses indicated populations ZJ,CN,HN,WC and TH might have experienced population expansion,and the time of expansion of Chinese populations could be 0.781-0.725 Ma BP,whereas the time of expansion of population TH could be 0.035 Ma BP.The phenomenon might occur in Pleistocene period with melting of glaciers,changes of climate warming and rising of sea level.