A composition for preventing or treating muscle diseases or improving muscular function, containing fucosterol, Sargassum fulvellum, a Sargassum fulvellum dried powder, a Sargassum fulvellum extract, Sargassum fusiforme, a Sargassum fusiforme dried powder or a Sargassum fusiforme extract is disclosed. Fucosterol, Sargassum fulvellum, a Sargassum fulvellum dried powder, a Sargassum fulvellum extract, Sargassum fusiforme, a Sargassum fusiforme dried powder or a Sargassum fusiforme extract increases the protein expression of p-mTOR, which is a main gene involved in muscle protein synthesis, inhibits the mRNA expression of MuRF-1 and atrogin-1 involved in muscle protein degradation, and increases the mRNA expression of MyoD and myogenin involved in muscle differentiation, thereby having an effect of remarkably increasing muscular function. In addition, the composition can be safely used without side effects, thereby being usable in medicine, food, cosmetic products, livestock feed, a feed additive and the like.