As a top predator, the blue marlin (Makaira nigricans) plays an important ecological and economic role in pelagic ecosystems. The aim of this study was to explore the feeding ecology of blue marlin and the composition of their prey species off eastern Taiwan by examining their stomach contents. To understand the feeding habits of the blue marlin, we used the index of relative importance (IRI) to examine the prey composition. The results showed that the prey compositions in the stomachs of blue marlin were different among the differing size classes. The large blue marlin with high mobility preferred to feed on mesopelagic fishes, while the small blue marlin preferred to feed on small forage fishes. Otherwise, prey IRI values varied across the different seasons. The most common prey species in spring were Auxis rochei rochei and Exocoetidae (IRI = 1,144 and 806). The most important prey species in summer were Trichiuridae and Cephalopoda (IRI = 1,913 and 1,078). Carangidae and Scombridae were the major prey species in autumn (IRI = 2,087 and 1,538), while Cephalopoda and Katsuwonus pelamis were found most commonly in winter (IRI = 1,465 and 828). The results suggested that the blue marlin is an opportunistic feeder that feeds on the dominant pelagic fishes in each season.