Sagassum yezoense (Yamade) Yoshida et Konno is a large fucacean brown alga that occurs in Korea and Japan. The species is one of the most common and abundant marine algae on the east coast of Korea. Investigation on seasonal variation of growth and decline of thalli, reproduction, and morphological features of the species was carried out bimonthly for a target population in Homigot, Pohang, from April 2007 to February 2008. Ten quadrats (50 cm × 50 cm) per every visit were randomly put on S. yezoense beds and, after measuring its coverage, all thalli in the quadrats were collected, and examined for morphology. Surface seawater temperature was in a range of 9oC in February and 22.6oC in September. Coverage was high (74%) in April and low (15%) in October. The average length of thalli was large with 78.2 cm in June and was low 8.6 cm in September. Most of thalli were fertile in June, having male and female receptacles. S. yezoense beds gradually declined from August and small juveniles occurred from October. However, the length of leaves was shorter in June (1.7 cm) than (3.5 cm) in October, and the same was true for leaf width. S. yezoense had germination period from September, and growth period from December, and reproductive period from June. The Pearson correlation coefficient between length of thallus and surface seawater temperature was negative. As a low temperature species on the east coast, S. yezoense will be used as an indicator species for detecting warming of coastal waters in Korea.