In the contemporary ocean, diatoms are responsible for about 40% of the global oceanic primary production. However, in the high nutrient low chlorophyll (HNLC) regions, the growth of diatoms is often limited by iron (Fe) and/or light that both play an important role in photosynthesis. Previous studies have shown that Fe availability can be affected by light intensity and seawater pH, and also CO2 concentration and light intensity can affect the Fe requirement of diatoms. Hence, the ongoing seawater acidification, as anthropogenic CO2 dissolves into the ocean, coupled with the increasing light intensity, as warming augments water column stratification, will likely lead to significant effects on Fe-limited diatoms in the high altitude regions. In this study, we used Phaeodactylum tricornutum as a model organism to examine how Fe-limited diatoms respond to ocean acidification under different light intensities by analyzing transcription and expression of key genes and proteins involved in Fe acquisition, storage and photosynthesis. The preliminary results showed that under low light conditions, high CO2 significantly improved Fe use efficiency, as reflected by the increased ratios of growth rate to Fe quota, and enhanced photosynthesis (e.g., Chla content, Fv/Fm, and PsaC, PsbA and RbcL expression) of P. tricornutum especially under Fe limitation. Further study is in progress to evaluate the effects of acidification on Fe and C uptake rates and key gene transcription under different light and Fe conditions. Overall, our study would provide an insight into the response of diatoms to ocean acidification in light and Fe co-limited environments.