New information on catch-at-age composition and length–weight relationships is presented for Pacific northern bluefin tuna caught by Taiwanese small-scale longliners in the southwestern North Pacific. The fork length – eviscerated weight relationship of Pacific northern bluefin tuna, Thunnus thynnus, caught off southwestern North Pacific (off the Taiwan coast) was determined and compared with relationships previously reported by various studies for different waters. The best representative of this relationship is W = 0.000023058L2.9342, where W is the eviscerated weight (kg) and L the fork length (cm). The eviscerated weights estimated from this relationship were multiplied by a scaling factor of 1.112 to obtain the estimated round weight. The visual comparison of weight-at-length showed that there were almost no differences between the relationships previously reported for the northwestern Pacific, the Sea of Japan and the present study. This indicated that Pacific northern bluefin tuna from those three waters could be of the same stock. Further assessment studies should combine information from those waters.