Marine diatoms contribute approximately 20% of the global primary production through photosynthesis, which are facing the challenges of climate changes. Ocean warming, as one of the most concerned anthropogenic influences, can accelerate inorganic carbon diffusion, reduce the inorganic carbon concentration, and affect the activity of the enzymes involved in cell metabolism. However, there is scant knowledge about the influences of ocean warming on photosynthetic carbon fixation in diatoms. Here, we cultured the model diatoms Phaeodactylum tricornutum and Thalassiosira pseudonana under 19 and 22 oC to determine how ocean warming influences CO₂ concentrating mechanisms, by measuring photosynthesis-dissolved inorganic carbon (DIC) curves. We found that high temperature enhanced the affinity of DIC and carbon use efficiency for photosynthesis of both P. tricornutum and T. pseudonana. We also examined inorganic carbon uptake from the surrounding environment, using DIDS (inhibitor of HCO₃^- transporters), AZ (inhibitor of periplasmic carbonic anhydrases) and qRT-PCR of critical genes involved in DIC uptake from the surrounding environment. For both P. tricornutum and T. pseudonana, the photosynthetic inhibition ratios of DIDS reduced as temperature increased, indicating that high temperature reduced the uptake ratio of HCO₃^- : DIC. Furthermore, the transcriptional levels of genes encoding plasma-type HCO₃^- transporters also down-regulated in response to high temperature. According to AZ inhibition experiments, we observed that the activity of periplasmic carbonic anhydrases increased in T. pseudonana under high temperature conditions, and was deficiency in P. tricornutum under either temperature conditions. Our results suggest that diatoms could enhance CO₂ concentrating mechanisms by accelerating CO₂ uptake under the future ocean warming scenarios.