Within the tropical and subtropical oceans, tuna forage opportunistically on a wide variety of prey. However, little is known about the trophic ecology of the smallest size classes which play an important role in stock assessments and fisheries management. The yellowfin tuna (Thunnus albacares) is one of the most abundant small tuna species in the waters off eastern Taiwan. This species is commercially exploited in the eastern Taiwan coast by traditional fisheries. The foraging behavior of juvenile yellowfin tuna (31.8–95.0 cm FL), collected from eastern coast of Taiwan was studied using stomach contents analyses. Juvenile yellowfin tuna changed their diets significantly between 45 and 50 cm fork length (ca. 1.5–2.0 kg). Smallest size classes fed on planktonic organisms inhabiting the shallow mixed layer, primarily larval stomatopod and amphipod, whereas larger tuna fed on larger teleosts and squids. The stable isotope analyses will be applied to examine food web dynamics of yellowfin tuna off eastern Taiwan waters.