The reproduction biology of longtail tuna, Thunnus tonggol, was investigated by 215 fish in the waters off eastern Taiwan during December 2005 thought April 2008. Histological evaluations of ovaries from 106 females and testis from 107 males provided the foundation for the estimates of length-specific reproductive characteristics. The estimated length at which 50% of the male were mature was 45.35 cm. The estimated length at which the female were immature to be smaller than66.5 cm. Spawning was not observed in this study.