The rosy seabass (Doederleinia berycoides) is a commercial fishery targeted demersal species which lives in the northeastern, eastern and southwestern waters off Taiwan at depths ranging from 200 to 400 m. This study focused on the length composition and reproduction of rosy seabass specimens caught by bottom longline fishery off the eastern Taiwan waters. From May 2013 to September 2015, a total of 1058 rosy seabass were caught, and all of these specimens were measured for fork length (FL, 14.7~41.3 cm), body weight (BW, 70~1,100g) and gonad weight (GW). There were 721 female (31.8 ± 2.9 cm FL; 473.2 ± 133.6 g BW) and 337 male (24.2 ± 2.0 cm FL; 207.3 ± 46.0 g BW) specimens, for an overall sex ratio of 68.1 %. The average body size of the female specimens was larger than that of the males, and most of the fish with a FL larger than 28 cm were females. The relationship between BW and FL of female and male was BW = 4.09 × 10-2FL2.70 and BW = 9.52 × 10-2FL2.41, respectively. The monthly variation in the gonadosomatic index (GSI) showed that the value was high between August and October (for both females and males) and that it peaked in September. Therefore, it is postulated that the spawning period occurs between August and October.