Available ecological and fishery data of sailfin sandfish, Arctoscopus japonicus, was examined to ascertain its population dynamic parameters in Korean waters. The instantaneous coefficient of total mortality (Z) was estimated to be 1.361/yr, and annual survival rate (S) was 0.256. The estimated instantaneous coefficient of natural mortality (M) was 0.482/yr, and the instantaneous coefficient of fishing mortality (F) for recent years was calculated to be 0.879/yr from Z and M values. Age at first capture was estimated to be 1.958 years. These parameters, in conjunction with catch landings data between 1991 and 2008, were used to estimate annual biomass using a biomass-based cohort analysis. The biomass of A. japonicus was estimated at approximately 7,600 mt in 1991, but since 1994 decreased to below 4,000 mt by 2001. After 2002 they started to increase gradually, and showed the level of more than 5,000 mt in recent years.