The radioprotect effects of fractinated fucoidan, derived from Undaria pinnatifida sporophylls, were examined in lung injured rats treated with partial body irradiation. The right lung of Sprague-Dawley rats, eight separately grouped by including radiation only control group (ROG) and fucoidan+radiation treated group (FRG), were treated with various fraction of Undaria-derived fucoidan every other day by intraperitoneal injection 6 days prior to irradiation and during monitoring at 24 hours, 48 hours, 2 weeks, 4 weeks and 12-weeks post-irradiation of 6 Mev/2 Gy. The rats were euthanized at completion of the treatment. Tissue samples from the injured lung, fixed in formaldehyde using lung perfusion prior to extraction, were H/E stained for histological analysis using optical microscopy. Interstitial congestion (IC), hemorrhage in alveolar sac (HM), number of macrophage (MP) and alveolar wall thickness (AWT) as a measure of early indication of fibrosis were measured. AWTs in 24 hrs post-irradiation rats increased in comparison with $2.49{pm}0.06{mu}m$ of control group by $3.67{pm}0.09{mu}m$, $3.21{pm}0.08{mu}m$ (p=0.013), $2.98{pm}0.08{mu}m$ (p=0.00) in ROG, F1-fucoidan+radiation, and F3-fucoidan+radiation group, respectively. AWT of the ROG was further increased by $4.30{pm}0.13{mu}m$ in the 12 weeks post-irradiation group, but AWT on average was $2.56{pm}0.05{mu}m$ in the FRG. MP in the 24 hrs post-irradiation group markedly increased in comparison with $2.6{pm}0.34/0.14;mm^2$ of the control group by $8.0{pm}1.48/0.14;mm^2$ in the ROG, but it was only $3.6{pm}0.48/0.14;mm^2$ in F3-FRG. MP in the 12 weeks post-irradiation group was $7.2{pm}1.28/0.14;mm^2$ in ROG, but it was $2.8{pm}0.37/0.14;mm^2$ or $2.4{pm}0.4/0.14;mm^2$ (p<0.05) in the FRG. In addition, increased IC and HM in ROG were relatively smaller in FRG of the 24 hr and 12 weeks post-irradiation rats. In conclusion, Undaria pinnatifida sporophylls-derived fucoidan fractions exhibited radioprotectant activity on injured rat lung subjected to X-ray irradiation. However, success was variable according to the fractions and also time of injection post injury.