The Philippines occupy the north of coral triangle, a huge area overlapping the Indian Ocean and the Pacific Ocean, and is famous for its extremely rich marine biodiversity. The taxonomical study of seaweeds started on the year 1750 and towards the end of the 20th century. In Ilocos Sur, a comprehensive study on the marine benthic algae was made by Domingo in 1988.In this study, he reported 103 species, with 91 species reported for the first time for the province.As to this date, no taxonomical study made in San Francisco-Canaoay. This study used site description, Exhaustive Line Transect Method.The macroalgae recorded in San Francisco-Canaoay in the open area of the coastlines includeone genus&sevenspecies of Phaeophyta, one genus&one species of Chlorophyta, and one genus &nine species of Rhodophyta. The larger number of Rhodophyta can be attributed to the presence of abundant nutrients in the area and the luminosity of light of 35, 000 Lux value, which was favorable for the photosynthetic macroalgae.The specimens identified,58% were Phaeophyta, 34% were Chlorophyta, and 8% were Rhodophyta.There were more red algaes (Rhodophyta) in the open area due to the nature of these plant-like protists to thrive in running water with many nutrients. The number of brown algaes (phaeophyta) are not affected by the conditions of the water because of their toughness as compared to the aforementioned nature of red algaes.