Diversity, abundance and distribution of intertidal macroalgae were investigated in relation to environmental conditions (site, shore level, salinity, temperature, NO 3- and ) in Sirinart Marine National Park, Phuket province, Thailand, from January to November 2004. The shores were divided into 3 categories by shore level: upper, mid and lower shores; and 3 sampling sites: sites 1, 2 and 3. A total of 52 species of macroalgae were recorded. Nine species are considered to be new records for the Thai marine flora. The average diversity index of macroalgae differed slightly among sites: 0.92±0.07 (mean±SE), 1.00±0.13 and 0.94±0.16 for site 1, site 2 and site 3, respectively. There was significant difference in abundance of macroalgae among sites and seasons. Lyngbya majuscula and Padina australis were the most abundant species. Their percentage covers were 31.66±7.21% and 27.77±4.67% in site 2 at mid shore level during March 2004 and in site 3 at mid shore level during November 2004, respectively.