Abstract: In the present work, transgenic cucumber seedlings over expressing CsRCA and wild-type cucumber seedlings ‘08-1’at three-leaf stage exposed to high temperature (40 ℃, PFD 600 μmol· m^-2 · s^-1) were used to study the regulatory mechanism of photosynthesis by CsRCA. The results showed that the mRNA abundance of rbcL and rbcS as well as the activities of ribulose bisphosphate carboxylic enzyme (Rubisco) and Rubisco activase (RCA) were significantly higher in CsRCA over-expressing cucumber seedlings than in wild type (WT). Following 2-h exposure to high temperature, a notable decrease was observed in photosynthetic rate (P_n), photochemical perfor-mance index based on the absorption of light energy (PI_ABS), activities of Rubisco and RCA as well as the relative expression of rbcL, rbcS and CsRCA in both wild-type cucumber seedlings and transgenic cucumber seedlings. It was found that high temperature stress led to higher W_k, a parameter of chlorophyll (Chl) a fluorescence OJIP curve. Furthermore, high temperature greatly reduced the efficiency of electron transfer along the electron transport chain beyond Q_A(ψ₀) and the quantum yield for electron transport (φ_E0), indicating that PSII oxygen complexes (OEC) and electron transport chain downstream Q_A were inhibited by high temperature. However, the inhibition could be alleviated by over expressing CsRCA in cucumber seedlings. Taken together, our data suggested that over expressing CsRCA improves photosynthesis in cucumber seedlings under high temperature stress by enhancing activities of the Rubisco and RCA, and maintaining the number of active reaction centers.