Seasonal and partial variations in the chemical composition of the brown seaweed Sargassum microcystum were studied. Brown seaweed was collected in rainy and dry seasons. Fresh seaweed samples were washed and then dried under the sunshine until the weight of seaweed was immutable. After being dried, whole dry brown seaweed was exhaustively ground to powder form and then extracted in 0.1N HCl for 3 h. In another experiment, upon arrival, fresh S. microcystum collecting in dry season was separated into different parts including blades, stipes and vesicles then these parts were individually extracted with the same method described above. Yields of polysaccharide were recorded and all extracts were then analyzed for chemical composition to evaluate seasonal and partial variations of nutritional values. The results found that S. microcystum in rainy season contained higher yield of polysaccharide than that in dry season. Yields of polysaccharide in S. microcystum were 40.2±18% and 18.9±0.5% for rainy and dry seasons, respectively. In rainy season, S. microcystum exhibited significant higher protein, phosphate, glucose and L-fucose concentrations than those in dry season. However, sulfate contents and phlorotannin concentrations containing in polysaccharide of S. microcystum were insignificant difference between rainy and dry seasons (p> 0.05). For the nutritional values in different parts, there was no significant difference in yields of polysaccharide extracted from blades, stipes and vesicles of S. microcystum. Interestingly, among different parts of S. microcystum, the significant higher protein and phosphate concentrations in blades were found (p< 0.05). Stipes contained significantly lower glucose and L-fucose, whereas phlorotannin concentrations showed insignificant (p> 0.05).