Linear discriminant functions for identifying three scombrid fish species at the at the juveniles stage were derived. The Juveniles of the Bluefin tuna（n=16）reared at Kinki University in June 2012 and those of the long-tailed tuna（n=8）and bullet Tuna（n=13）collected by surface trawl in the western part of the Japan Sea on Tenyo-Maru, a fisheries training vessel of National Fisheries University, in August 2012, were used as the Juvenile samples. We selected a total of nine measuring regions for these juveniles specimens, used as the explaining variables the values of the morphological measurement at each of these regions, which were standardized by the fork length, and obtained the discriminant. The accuracy rate of the discriminant was 94.4%（p=0.00）for the Thunnus fishes（bluefin tuna and longtailed tuna）and bullet Tuna and 91.3%（p=0.02）for bluefin tuna and long-tailed tuna. The result of the morphological measurement supported the contribution ratio for the discriminant. In addition, we maked a simplified discriminant by using “body depth,” the measured region with a high contribution ratio, for the discrimination between the Thunns fishes and bullet Tuna and “body depth” and “length between the base of the pelvic fin and the fork of the tail fin” for the discrimination between bluefin tuna and longtailed tuna.