This paper studied the dynamics of reproductive allocation (RA) of Sargassum thunbergii during its sexual reproductive season and the related environmental factors at the Taiping Cape of Yellow Sea. The sexual reproduction of S. thunbergii initiated in early June, peaked in mid July when the sea water temperature was about 22 degrees C (the mean proportion of biomass allocated to reproductive organs on July 19 was 76.7%), and ended in late August. The RA had a significant linear correlation with the average length of thallus branches (r = 0.855, P < 0.01). The thalli with a length less than 10 cm showed a lower RA in the whole sexual reproductive season, while the thalli longer than 10 cm had a RA up to averagely 70.0% at the peak maturing stage. UNIANOVA analysis showed that both tidal level and wave strength had significant effects on the RA of S. thunbergii (tidal level: F = 175.62, P < 0.01; wave strength: F = 95.35, P < 0.01), and there was a significant interaction between tidal level and wave strength (F = 9.14, P < 0.05). The sizes of the effects were in the order of tidal level > wave strength > tidal level x wave strength.