The effects of high temperature on the seedlings of cork oak were studied with respect to growth, and proline accumulation. Within this context, this work focuses on the impact of short and repetitive periods of high temperatures (38°C, 40°C, 42°C and 44°C) on certain morphological and biochemical aspects of the cork oak seedlings. Height, foliar area and number of leaves were found to be positively affected with the temperatures. Proline accumulation was found in leaves, stems and roots after thermal stress treatment. A preferential accumulation of proline was found in root system, with a maximum at 44°C. Thus, the level of proline accumulation was related to the degree of thermal stress tolerance.