Abstract: Cucumber seedlings (Cucumis sativus Jinyou 35) were used to study the effects of high temperature (HT: 42 ℃/32 ℃) and sub-high temperature (SHT: 35 ℃/25 ℃) on its photosynthesis and growth. The results showed that the growth of cucumber seedlings was dramatically inhibited by the high and sub-high temperature stresses. The photosynthetic rate (P_n) was gradually reduced, while intercellular CO₂ concentration (C_i) was increased as heat stress lasted. Under heat stress, stomatal conductance (g_s), transpiration rate (T_r), photorespiration rate (P_r) and dark respiration rate (D_r) showed a trend from rise to decline in cucumber seedlings, which implied that heat-induced decline of photosynthesis was mainly due to non-stomatal limitation. Maximal photochemical efficiency of PSⅡ in darkness (F_v/F_m), actual photochemical efficiency (Φ_PSⅡ), photochemical quenching (q_P) and electron transport rate (ETR) were severely hampered, while initial fluorescence (F_o) and non-chemical quenching (NPQ) were increased as a result of high and sub-high temperature stresses. Under extended high temperature stress, the activities of RuBP carboxylase (RuBPCase) and Rubisco activase (RCA) as well as the mRNA abundance of Rubisco and RCA were in the trend of decrease, while they were reduced 3 days following the sub-high temperature treatment. The activities and mRNA expressions of sedoheptulose-1,7-bisphosphatase (SBPase) and fructose 1,6-bisphosphate aldolase (FBA) increased initially, but decreased afterwards under heat stress. Taken together, our data suggested that short-term sub-high temperature did not cause photoinhibition under optimal light conditions, however, high temperature led to severe damage to PSⅡ reaction center in cucumber seedlings. The photosynthetic enzymes were induced by high temperature stress and the induction was affected by temperature and stress duration.