Ecological characteristics of Undariopsis peterseniana, an endangered brown alga, were investigated from January to December 2007 at Jeju Island, Korea. The U. peterseniana population formed dense patches at depths of 12~17 m depth. Growth in length of the alga reached a maximum of $168{pm}36cm$ at June. Sorus were observed from May to August, and a maturation peak was detected in June, when bottom seawater temperatures were around $18^{circ}C$. After zoospore release, the alga bleached and disappeared after August. Developmental initiation of sporophytes occurred at temperature above $15^{circ}C$, and sorus formation required approximately 236 degree-days. This is the first study to examine a U. peterseniana population throughout the entire year in a natural habitat.