The authors investigated the spatiotemporal distribution of intertidal benthic macro-algae and their diversity in 10 sections of southern Zhejiang province in autumn 2006 and spring 2007. The results showed that 61 species of benthic macro-algae were collected, including eight Chlorophyta, 16 Phaeophyta, 36 Rhodophyta and one Cyanophyta. Flora characteristic of the benthic alga distribution was very obvious in the inter-tide of southern Zhejiang, and it also changed obviously with seasons in species composition and biomass of benthic algae. More species were found in spring (53 species) than in autumn (25 species), among which only 17 species existed in both of these two seasons. The average biomass of intertidal benthic marco-algae was 540.16g•m-2 in southern Zhejiang, and there was more biomass in spring (722.18g•m-2) than in autumn (358.14g•m-2). As for the vertical distribution of biomass, it was also different in the 10 sections during the two seasons. In autumn, the biomass of intertidal benthic marco-algae was in the order of low intertidal zone (739.45g•m-2)> middle intertidal zone (334.95g•m-2)> high intertidal zone (0g•m-2); in spring it was sequenced as middle intertidal zone (1 943.30g•m-2)> low intertidal (965.03g•m-2) > high intertidal zone (290.41g•m-2). There was something in common for the dominant species in the two seasons in terms of dominance, but their functions in the community changed greatly with seasons. For example, Sargassum thunbergii (Mert.) O. Kuntze was the first dominant species in spring, but it was the second in autumn. The averages of Shannon-winner index, Margalef’s species richness index and Pielou’s evenness index were 1.38±0.66, 0.70±0.38 and 0.93±0.41 in autumn, and 1.65±1.09, 1.07±0.63, 0.80±0.38 in spring, respectively. There was great difference in diversity of benthic marco-algae among sections, which was related to the stability and complexity of benthic marco-algae in southern Zhejiang, as certified by the cluster analysis.