Seaweed forests of Eisenia bicyclis Setchell and Ecklonia cava Kjellman are broadly distributed along the rocky coastal area of central Honshu Island, Japan, facing the Pacific Ocean. We aimed to clarify a process of zonation which are consisted of the two plants. We measured attaching force of holdfast to a substratum in a field and fluid forces on them in a laboratory. Attaching force of their holdfast depended on a condition of substratum surface as same as adult plants. Dominant fluid force exerted on juvenile plants was a drag force different from adult plants because of little development of stipe part. For juvenile as well as adlut plants, attaching force of their holdfast was almost higher than maximum fluid force in situ by using model data.