Global warming has increased the frequency and duration of high temperature (HT) stress. Photosynthesis determines yield in maize and is extremely HT sensitive. The effects of HT on photosynthesis in maize leaves have been strongly examined under controlled conditions. Here, to explore the mechanism and primary inhibitory sites of HT to photosynthesis, the HT sensitivity of photosynthesis in XY335 and ZD958 maize hybrids was systematically studied in field by multiple methods. HT decreased leaf area and photosynthetic rate of unit leaf area and hence limited growth. HT disrupted chloroplast and mitochondrial membrane structure, possibly delaying photosynthetic recovery after HT. These changes were greater in XY335 than ZD958. Stomatal conductance decreased significantly under HT, and this did not restrict CO2 fixation but may weaken the heat dissipation through transpiration. HT caused photoinhibition of PSII but not PSI. HT damaged both the oxygen‐evolving complex, located at donor side of PSII, and electron transfer from QA to QB, located at acceptor side of PSII. Interference of electron transfer from QA to QB caused by degradation of QB‐binding (D1) protein was the primary site of PSII inhibition by HT in maize leaves. The different stomatal behaviour and photoinhibition sites under HT between maize and wheat were discussed.