Abstract: The round scad，Decapterus maruadsi， is a commercially important species in northern South China Sea. However， the catches of the species has declined severely. Thus， there is a strong need to study the population genetic of D. maruadsi in northern South China Sea for the effective conservation and sustainable fishery management. In this study， genetic diversity and genetic structure of D. maruadsi in northern South China Sea were analyzed based on 203 individuals of 9 populations using mitochondrial DNA cytochrome b（Cyt b） sequence. A 722 bp segment of mtDNA Cyt b gene was sequenced， which revealed 52 polymorphic sites and defined 25 haplotypes. The haplotype diversity and nucleotide diversity were 0.577±0.036 and 0.001 55±0.001 12， respectively， which indicated that genetic diversity of D. maruadsiin northern South China Sea was low to moderate. Five populations in the east of Leizhou Peninsula and Hainan Island showed higher level genetic diversity than 4 populations in the west. No clustering corresponding to sampling localities was found in Bayesian inference tree， neighborjoining tree， and haplotype network. The haplotype network was starlike with two dominant haplotypes surrounded by other lowfrequency haplotypes. The pairwise Fst values between nine populations ranged from -0.077 to 0.018 and showed no significant genetic difference. Analyses of molecular variance （AMOVA） revealed that all genetic variations were attributable to the variations among individuals within populations. Both Tajima's D（D=-2.602，p=0） and Fu's Fs（Fs=-23.376，p=0） yielded significant negative values. Unimodal distributions were observed for the total population. The results suggested D. maruadsi from northern South China Sea had experienced a population expansion at about 29 000 years ago， which was responsible for the genetic diversity model with highh value and low π value. Genetic structure analysis revealed that there was no significant genetic differentiation among nine populations， which indicated that D. maruadsi from northern South China Sea was consistent with the hypothesis of a panmictic population. However， the obvious geographic tendency of genetic diversity was detected among nine populations. The inconsistent reminded us to cautiously manage the populations of D. maruadsi in northern South China Sea as a ‘one stock’．