Effects of light qualities (white light, red light and blue light) on the growth and rhizoid development of Laminaria japonica Aresch young sporophyte were investigated. During the culture period of 56 days, the young sporophyte cultured in blue light had a frond size of 9530±125 μm length/1 350±49 μm width, about four folds greater than that in white light; and the young sporophytes’ growth was much delayed in red light, at a size of 580±63 μm length/135±11 μm width. However, the development of rhizoid showed little difference between different light qualities. The ratios of rhizoid length to frond length were the highest in red light and lowest in blue light, which all decreased as the cultures prolonged. Fucoxanthin, one of the major light harvesting pigments in Laminaria with absorption spectrum peaking in blue light region is highly effective in capturing blue light. Consequently, the more the blue light irradiance was provided, the more the light energy was captured, resulting in the higher photosynthetic rate. Because the rhizoid is differentiated from the Laminaria frond and possesses degraded photosynthetic capacity, it was less reactive to light qualities than the frond and showed similar development under the three lights.