The aims of the research were to find out nutritional composition of seaweed Sargassum crassifolium J. Agardh i. e. concentration of protein,
mineral elements (Ca, Fe, and P), vitamin C, vitamin A, lipid, fatty acids and alginates. S. crassifolium is a species of brown seaweed that is consumed as source of food, however, it have not optimally used due to the nutritional composition information does not complete yet. The measurement of protein concentration was done according to Lowry method, while amino acids concentration was measured using High Performance Liquid Chromatography (HPLC). Mineral (ash) was measured by dry ash processing, and mineral elements of Ca, Fe, and P were measured using atomic absorption spectrophotometer (AAS) and UV-Vis spectrophotometer. Vitamin C concentration was measured by titration method, while vitamin A was measured using UV-Vis spectrophotometer. Lipid was measured by extraction method using soxhlet, fatty acids by fatty acids methyl esters (FAMEs) method, and alginates were measured by extraction method. The results indicate that the thallus of S. crassifolium contain protein in the average of 5.19% (w/w), and 17 amino acids (in “mol amino acid/g wet weight) varies from 13.77 of glutamic acid to 0.83 for hydroxilicine concentration. Mineral/ash content was 36.93% (w/w), Ca: 1540.66 mg/100 g, Fe: 132.65 mg/100 g, P: 474.03 mg/100 g, vitamin C: 49.01 mg/100 g, vitamin A: 489.11 “g RE/100 g, lipid: 1.63% (w/w), fatty acids concentrations were: 1.45%, 3.53%, 29.49%, 4.10%, 13.78%, 33.58%, 5.94% for lauric acid, meristic acid, palmitic acid, palmitoleic acid, oleic acid, linoleic acid, and linolenic acid subsequently. The concentration of alginates was 37.91% (w/w). Keywords: nutritional composition, seaweed, Sargassum crassifolium J. Agardh.