THE PRESENT study was carried out to investigate the effect of two … temperature degrees (30&40°C) on physiological and biochemical aspects of barley seedlings and their impact on growth and yield. Plant height, the number of tillers, fresh weight, dry weight, flag leaf area were generally reduced following exposure to heat stress ranging between 2 and 8 hours., while relative water content was increased indicating attempts by the plant to resist temperature elevation. The contents of chlorophyll a and chlorophyll b decreased significantly in leaves exposed to all treatments of heat stress while carotenoids decreased as the stress duration was prolonged. The correlation was evident between growth, some yield parameters (peduncle length, biological yield, spike length, the number of spike per plant, kernels weight) and yield index. Changes in SDS protein profiles indicated the occurrence of a number of biochemical responses; some of them were positively associated with heat stress, while the others were negatively associated with heat stress.