This study focuses on the effects of dyeing conditions and mordanting conditions, changes of colorfastness, antibacterial properties and UV protective of silk fabric dyed with Laminaria japonica extracts. A natural colorant was extracted from Laminaria japonica using distilled water as extractants. According to the results, maximum dye uptake (K/S) were obtained at 100% V/V colorant concentration, $80^{circ}C$, 60 min and pH 2. Silk fabric was dyed with Laminaria japonica extract at $80^{circ}C$ for 60 min with pre-treatment or post-treatment of various metal salts as mordants. The Laminaria japonica extracts produced yellow hue on silk fabric in pre-mordanting and manifested green yellow hue mordanted with $CuSO_4$ in post-mordanting method. Mordant $CuSO_4$ for silk fabric was found to give good light fastness (rating 4). UV protection property did not increase significantly upon mordanting.