Abstract: In order to study the origin and characteristic of the floating Sargassum horneri in the Yellow Sea, floating S. horneri samples were collected from the northern, southern Yellow Sea and the coastal water of Qingdao. The attached samples on the aquaculture rafts along the coast of Changdao were also investigated seasonally. The study showed that the plant morphology of floating and attached S. horneri had distinct seasonal patterns. In November, there was no significant difference of the leaves and gas-filled vesicles between the floating and attached samples. Form April to June, they had obvious morphological differentiation, such as leaves and vesicles of the attached S. horneri were growing while those of the floating samples tended to decrease. The receptacles of the floating S. horneri was obviously less and slimmer than that of the attached ones. Meanwhile, one nuclear (ITS2) and 4 mitchondrial gene fragments (cox3, rnl-atp9, rpl5-rps3, cob-cox2) were selected to investigate the genetic diversity among these S. horneri samples in this study. The results showed that there were 2 polymorphic loci and 5 genotypes detected from ITS2 sequences, 9 polymorphic loci and 5 haplotypes based on the mitchondrial genes in these samples. The inter-population genetic distances among the geographic samples and between floating and attached samples were not significantly different with those intra-population genetic distances, which suggested these samples could be from the same genetic population.