Abstract: Influences of nitrogen (N) and phosphorus (P) ratios on photosynthetic inorganic carbon utilization and carbonic anhydrase activity in marine diatom Phaeodactylum tricornutum were investigated. The results showed that the growth rate in the algae grown under the medium with N:P=16:1 was the largest. Higher or lower than N:P=16:1 resulted in a significant decrease in growth rate, which indicated that the optimum growth was limited by nitrogen or phosphorus supply. Chlorophyll a content was reduced by 30.1% and 47.6% in N-limited cells (N:P=4:1 or 1:1), by 39.1% and 52.4% in P-limited cells (N:P=64:1 or 256:1), however N or P limitation showed no significant affect on the content of chlorophyll c. Compared with nutrient-replete cultures, severe N or P-limited cultures (N:P=1:1 or 256:1) resulted in a decrease by 39.7% and 48.0% in light-saturated photosynthetic rate. Nutrient-limited conditions also caused a significant decline in photosynthetic efficiency and dark respiration rate. K0.5CO2 decreased by 30% in P-limited conditions, which indicated P limitation could increase the affinity of the algal cell for CO2. Plasma membrane redox rate was lower in nutrient limitation than that in nutrient replete. Extracellular carbonic anhydrase activity was reduced by 50% and 37.5% in N-limited condition, by 22.3% and 42.1% in P-limited conditions relative to nutrient-replete conditions. Intracellular carbonic anhydrase activity was 36.5% and 42.9% lower in severe N or P limitation than in nutrient-replete cultures. The above results showed that this alga grown under conditions of nutrient limitation (N or P) could maintain a moderate growth by adjusting chlorophyll content, the mode of inorganic carbon utilization and carbonic anhydrase activity.