The present study provides new insights into the growth of brown algal cell wall by showing that cell wall polysaccharide plays an important role in the process of growth consider- ing the physicochemical characteristic of young and old Cladosiphon okamuranus. To determine its structural variation in detail, the cell wall was sequentially fractionated into five fractions: hot water (HW), ammonium oxalate, hemicellulose-I (HC–I), HC-II, and cellulose, and analyzed physicochemically. Results showed that almost 80% of the total recovery cell wall from both young and old thalli was HW and HC-I contained mainly fucoidan composed of Fuc, GlcA, and sulfate in molar ratios of 1.0: 0.3: 0.6~0.7 and 1.0: 0.3: 0.2~0.3, respectively. Fucoidan in HW was a highly sulfated matrix polysaccharide abundance in young thalli, while fucoidan in HC-I was rich in old thalli and functions as hemicellulose in land plants, crosslink with cellulose and strengthen the cell wall. We found that HW and HC-I were particularly involved in the growth and strength of old thalli appeared to be due to the deposition of HC-I and reducing of water content during the growth process.