Stable isotope technique has been widely applied for identification of animal food sources and reconstruction of food chains and food webs in marine ecosystems, but it has been sparingly used in the research on feeding habits of marine mammals. Compared to the traditional stomach content analysis, which is lengthy, requires a large number of samples, and can only reflect the present state of the sampled organism, the stable isotope technique can reveal its extended biological activity, accurately measure biological source of nutrition, and truthfully identify the biological relationship among populations and the energy flow of the entire ecosystem. Various studies have applied the stable isotope method to investigate feeding in marine mammals such as pinnipeds, polar bears, and others. At present, research of finless porpoise has focused on morphology, behavior, age identification, population dynamics, etc. The previous diet studies mainly use the traditional methods to examine stomach content, whereas feeding studies of the finless porpoise in Liaodong Bay using stable isotopes have not been reported yet. Finless porpoise (genus Neophocaena) is a coastal, small-teethed cetacean inhabiting estuaries and rivers. It is broadly distributed in China, from the northern to the southern coastal areas and into the Yangtze River. The finless porpoise, which is listed as vulnerable in the IUCN list, includes Neophocaena phocaenoides and Neophocaena asiaeorientalis, with two subspecies (Neophocaena asiaeorientalis ssp. asiaeorientalis and Neophocaena asiaeorientalis ssp. sunameri) within the latter species. In the present study, we evaluated carbon and nitrogen stable isotope ratios in samples obtained from finless porpoises that were stranded and died on the coast of the Liaodong Bay and from those obtained from main fishing catches from the same area from April to June in 2012. We also evaluated the isotope ratio in the potential prey organisms. The mean values of δ¹³C and δ¹⁵N in samples of finless porpoise were (-18.4 ± 0.3)‰ and (13.8 ± 0.4)‰, respectively, whereas δ¹³C and δ¹⁵N in samples of 28 kinds of potential prey organisms ranged from -19.5‰ to -17.0‰ and from 11.4‰ to 14.0‰, respectively. The trophic level of finless porpoise was 4.5, which was higher than that obtained by traditional stomach content analysis. The trophic level of the 28 types of potential forages ranged from 3.8 to 4.6. Fish was the main prey of the finless porpoise; the contribution rate of prey organisms, from the biggest to the smallest, was as follows: pelagic fishes > mesodemersal fishes > demersal fishes > cephalopoda > shrimps > crabs, with the average contribution rate of 43.9%, 18.2%, 13.1%, 10.0%, 8.8%, and 6.0%, respectively. There was no significant linear correlation between the ratio of δ¹³C and δ¹⁵N and the body length of finless porpoise; the carbon sources were stable and the nitrogen sources were complex. Overall, the application of stable isotope technique to study feeding habits in finless porpoise can improve our understanding of the biology of this species, and provide some essential information for its conservation.