Abstract: We investigated the composition of the stomach contents of 138 longnose lancetfish (Alepisaurus ferox) collected from September 2017 to January 2018 by the Chinese Longline Fisheries Observer Programme in the western tropical Pacific Ocean (165°45′E−178°00′W, 03°02′S−18°34′S). A total of 61 species from 44 families, belonging to six prey groups (Fish, Mollusca, Crustacea, Polychaeta, Cnidaria and Thaliacea) were identified. Cannibalism and plastic debris ingestion were also observed. The obtained values of Shannon-Wiener diversity index (H'), Pielou evenness index (J') and dominance index (D) of A. ferox's diet composition were 2.71, 0.67 and 0.12, respectively. The result of index of relative importance (IRI) analysis indicates that Mollusca was the dominant prey with IRI of 7 573.31 which accounted for 53.11% of total IRI. At family level, Sternoptychidae, Alepisauridae, Carinariidae, Pterotracheidae, Bolitaenidae, Onychoteuthidae, Oplophoridae, Platyscelidae, Eurytheneidae and Alciopidae were the most important preys of A. ferox.