This study investigated the protective effect of the butanol (BuOH) fraction from fermented Laminaria japonica extract (BFLJ) on AAPH-induced oxidative stress in porcine kidney epithelial cells (LLC-PK1 cells). L. japonica was fermented by Aspergillus oryzae at $35{pm}1^{circ}C$ for 72 h. Freeze-dried fermented L. japonica was extracted with distilled water, and the extracted solution was mixed with ethanol and then centrifuged. The supernatant was subjected to sequential fractionation with various solvents. The BuOH fraction was used in this study because it possessed the strongest antioxidant activity among the various solvent fractions. The BuOH fraction of fermented L. japonica had a protective effect against the AAPH-induced LLC-PK1 cells damage and increased cell viability while reducing lipid peroxidation formation and increased activities of antioxidant enzymes such as superoxide dismutase and glutathione peroxidase. The inhibitory effect of BFLJ on lipid peroxidation formation had a higher value of $0.11{pm}0.01nmol$ MDA at $100{mu}g/mL$ concentration in comparison with intact BuOH fraction showing $0.22{pm}0.08nmol$ MDA at the same concentration. Furthermore, BFLJ treatment increased glutathione concentration. GSH concentration in the cell treated with BFLJ of $100{mu}g/mL$ was $1.80pmol/L{ imes}10^5cells$. These results indicate that BFLJ protects the LLC-PK1 cells against AAPH-induced cell damage by inhibiting lipid peroxidation formation and increasing antioxidant enzyme activities and glutathione concentration.