This studies were carried out in summer season to increase high temperature tolerance using hydrogen peroxide treatments on cucumber in greenhouse. The water stress of cucumber in greenhouse by the hydrogen peroxide treatments showed as control>250 mM>500 mM treatments in order. The photosynthesis rate of cucumber at $30^{circ}C$ did not show difference with each hydrogen peroxide treatment in temperature controlled greenhouse. However, the photosynthesis rate of cucumber in the control and hydrogen peroxide treatments at $40^{circ}C$ was significantly different. The photosynthesis rate of cucumber in combined treatment with 1,000 $mg{cdot}L^{-1};CO_2$ supply and hydrogen peroxide was also higher than control, however, there was no different of photosynthesis in 250 mM and 500 mM treatment. The value of $F_v/F_m$ and $F_m/F_o$ of chlorophyll fluorescent in 500 mM hydrogen peroxide treatment at $40^{circ}C$ was highest. Also the activity of POD, the antioxidant enzyme, was higher with high hydrogen peroxide concentration than the other treatments. The high temperature limits for growth were $43^{circ}C$ in the control, $44^{circ}C$ in the 250 mM and $46^{circ}C$ in the 500 mM according to analyze chlorophyll fluorescent $F_o$. The high temperature tolerance in cucumber increased approximately $3^{circ}C$ by the hydrogen peroxide treatments under this experiment conditions.