A new floristic index, [I/H]RCP, for the estimation of seaweed flora depending on the seawater temperature, is proposed. The index is a ratio of [I]to[H]. [I] is the number of species including both of species with alternation of isomorphic generations and without alternation of generations, and [H] is the number of species including species with alternation of heteromorphic generations, for Chlorophyta, Phaeophyta and Rhodophyta. This new floristic index, [I/H]RCP, clearly demonstrates the positive relationships with seawater temperature which effects the change of seaweed flora.