Abstract: The effect of oxybenzone (BP-3) on the photosynthesis and respiration of wild Gracilaria lemaneiformis was investigated by determining the response of the rapid chlorophyll fluorescence, photosynthetic oxygen evolution rate, respiratory oxygen uptake rate, and active oxygen content in this alga exposed to 0 to 30 µmol/L of this reagent. Results showed that 5~30 mol/L BP-3 had a slight effect on respiration after 30 hours treatment without light, but could directly inhibit the electron transfer of PSII receptor side of photosynthesis in G. lemaneiformis. At 80 μmol/m²/s light, BP-3 caused an imbalance of photosynthetic light energy absorption and utilization, and further induced the over-production of reactive oxygen species. The inhibition was intensified with the increase of BP-3 concentration. The oxidative stress induced by BP-3 would aggravate the damage of PSII receptor side of photosynthesis, and further damage the respiration process and photosynthetic PSII donor side in the seaweed.