Abstract: To investigate the effects of seawater acidification, two major factors affecting seawater acidification were selected to investigate their influence on Skeletonema costatum. The growth rate, chlorophyll fluorescence parameters, particulate organic carbon (POC), particulate organic nitrogen (PON) contents and C/N ratio of the algae were measured under different conditions. The results showed that seawater acidification significantly inhibited the growth of S. costatum, with a growth rate of 0.86/d in the acidification group and 0.99/d in the control group, and that this inhibitory effect was mainly attributed to the effect of pCO₂. The acidification of seawater had no significant effect on the photosynthetic efficiency of S. costatum. Seawater acidification significantly increased POC and PON by 37.1% and 43.3%, respectively, with POC being affected by both pH and pCO_2, and PON being mainly affected by pCO₂. It is shown that seawater acidification was benefit to the carbon and nitrogen accumulation of S. costatum; however, no significant variation was observed in the C/N ratio among algal cells under the different acidification treatments.