In this study, length-frequency and sex ratio (proportion of females) of dolphinfish were analyzed based on 88,902 (54,717 were females and 34,185 were males) samples measured in Shingkang fish market during 2003 to 2018, and catch compositions and fishing grounds of dolphinfish longline fishery were also explored based on the catch records collected from sampling vessels operated during 2012 to 2018. The sex ratio shows that females are dominant, with a total ratio of 0.62. The proportion of male becomes higher than females when body length increased. The results of indicators related to length distributions indicated that the caches of dolphinfish in the eastern waters of Taiwan tend to be small in recent years, especially for females. The results of this study indicated that fishing exploitation might have led to the decreases in the body size in recent years. About 75% of the dolphinfish longline vessels operated in the waters off Shingkang from January to March. The longline vessels tended to operate in the southeastern waters from April to June and returned to offshores of Shingkang for operations from July to September. Then, they moved northward to waters off Hualien from October to December.