Abstract : Alginates are sulfated polysaccharides that existed in brown algae, possess various bioactivities and apply diversity in the fields of food, functional food, pharmaceuticals, and industry basing on their physico- chemistry characteristics. The paper focused on the impact of the growth times (January to June), the extracting temperature (70, 80, and 90oC) and time (1 to 9 hours), and Na2CO3 content (0.05 to 0.5%, compared to alginate content) on the viscosity and the average molecular weight of alginate which extracted from brown algae Sargassum polycystum grown in Southern Center, Vietnam. The results showed that the viscosity and the average molecular weight of alginate were affected by the extracting temperature and time, and Na2CO3 content (p<0.05) and changed according to the trend of the linear model. The effect of the extracting temperature on the viscosity and the average molecular weight of alginate was higher than the extracting time and Na2CO3 content. The difference in the viscosity and the average molecular weight of alginate occurred when the Na2CO3 content- to-sodium alginate ratio was larger and equal to 0.3/1 (w/w). Alginate content (33.8 mg sodium alginate/ g DW) of brown algae Sargassum polycystum was highest in May. The current study supports for the processing control of alginate preparation applicating into other fields such as food, functional food, pharmaceuticals and industry.