Effects of ammonia and phosphorus addition on Gracilarial emaneifomis photosynthetic rate were evaluated in terms of oxygen production and carbon acquisition, and the carbon sink function of this cultured seaweed was discussed. Iodimetry measurement for dissolved oxygen and total alkalinity (TA) determination for dissolved inorganic carbon (DIC) were taken to measure the rates of O2 production and DIC acquisition of G. lemaneifomis. During the experiment period, seaweed G. lemaneifomis was cultured in sealed synthetic glass containers under 4 different nutrient conditions (ADDN,10μmol/L NH4Cl; ADDP, 1.0μmol/L KH2PO4; ADDNP,10μmol/L NH4Cl+1.0μmol/L KH2PO4; NONP, without addition of nitrogen or phosphorus), in situ seawater as control. The results showed that from August to November, the added ammonia and phosphorus had no significant effect on RO (photosynthetic rate in oxygen production) of G. lemaneifomis; in November, the added ammonia and phosphorus significantly increased the RO of the seaweed. The added ammonia and phosphorus had no significant impact on RDIC. RCO2 ranged from 6.9 to 18.4μmol/g DW•h (average 11.3μmol/g DW•h). No significant difference in each condition but obvious seasonal variations were observed.RHCO-3ranged from 117.1 to 487.1μmol/g DW•h. ANOVA analysis revealed no significant difference in pCO2 for each treatment, but there were obvious seasonal variations in pCO2. Significant difference in photosynthetic quotient (PQ) was found among treatments and months; PQ (average) maximized (1.23) when ammonia was added, and the monthly average PQ was in the range of 0.59~1.50.