The influence of macronitrogen (NO3- and NH4+) addition with Ulva pertusa on dissolved inorganic carbon system in seawater was studied.The results indicate that p(CO2) and HCO3- concentration decrease significantly,while pH and CO32- concentration increase significantly. When the concentration of NO3- was less than 71 μmol/dm3 or NH4+ was less than 49.7 μmol/dm3,dissolved inorganic carbon (DIC) absorption rates by Ulva pertusa generally increased with the increasing of nitrogen concentration.The DIC decreased 151 μmol/dm3 with the addition of 71 μmol/dm3 NO3- and decreased 232 μmol/dm3 with the addition of 49.7μmol/dm3 NH4+ after the experiment compared with DIC measured without nitrogen addition. A significant negative-correlation was found between Ac(DIC) and growth rate (μ) of Ulva pertusa (r =-0.91,P ＜0.000 1,n=11).NH4+ had more influence on the species of inorganic carbon system than NO3-.