Abstract: Human behaviors contribute to global changes of enhanced solar ultraviolet radiation (UVR, 280-400 nm) and ocean acidification. Macroalgae plays an important role in global carbon cycle as primary producers; however, the effects of ocean acidification and UVR on macroalgae are uncertain. To investigate the combined effects of ocean acidification and UVR on growth and photochemical efficiency, the calcified Amphiroa sp. and non-calcified Cladophora sp. were utilized in the present study. The results showed that increasing CO2 concentration from 360 ppmv (the current CO2 concentration in the air) to 1000 ppmv inhibited the growth of Amphiroa sp. by 40.01%, but stimulated the growth of Cladophora sp. by 40.08%. We also observed that UVR sensitivity was increased with acidification, and that acidification amplified the effects of elevated CO2 concentration in regulating the growth of algae. This suggested that ocean acidification may increase the negative effect of UVR to algae. These results indicated species-dependent effects of ocean acidification and UVR.