Demersal fish is the main catch for traditional fishermen using longline fishing gear in Sentang Beach waters located in Serdang Bedagai Regency, North Sumatra Province. The high economic value of demersal fish is the focus of traditional fishermen to make it the main catch. This study aims to determine the composition of the catch and the abundance index of demersal fish caught by longline in the waters of Sentang Beach. The method used in this research is to use a random sampling method with the mechanism of joining fishermen to go to sea for one month every time fishermen go to sea in November 2022. The data analysis technique uses abundance index value analysis and multiple linear regression analysis. The results showed that the composition of demersal fish caught using longline fishing gear consisted of 5 species of fish namely Snapper (Lates calcarife), Threadfins (Eleutheronema tetradactylum), Pomfret (Pampus argenteus), Sagor catfish (Hexanematichthys sagor) and Mullet (Valamugil saheli) with the highest abundance index obtained by the mullet species. Obtained the highest relative relative abudance of fish found in Senangin species which is 100% and the lowest relative relative abudance of fish in the species of Snapper which is 1.14%.