The embryo spores of Laminaria japonica were cultured in artificial seawater with different N/P ratio to observe their early development. 80 percent of the spores cultivated in whole-ingredient artificial seawater developed normally, but the time of spore germination and gametophyte formation was lagged significantly and the amount of female gametes was significantly higher than that of male gametes (P<0.001), as compared to the control cultivated in natural seawater. Artificial seawater with different N/P ratio showed different effects on the spore germination rate. The artificial seawater with N/P > 15 decreased the spore germination rate, while that without N or P inhibited the spore germination significantly (P<0.001). It was concluded that L. japonica spores could develop normally in artificial seawater, but their germination rate was affected by the seawater N/P. The deficiency of N or P in artificial seawater would lag the time of gametophyte formation and affect the differentiation ratio of female-male gametes.