DEVELOPMENT OF some rice genotypes tolerant to heat stress during different growth stages is one adaptation strategy for mitigating the negative effects of global warming. To this end, assessing the trait variation and identifying suitable genetic donors is essential. We assessed the genetic variability for heat tolerance-related traits in 20 diverse cultivars under heat stress (HS) and non-stress conditions (NS). The experiments were laid out in a randomized complete block design with three replications in New Valley Research Station (south of Egypt) as heat stress conditions and Sakha Research Station (north of Egypt) as non-stress conditions to assess the effects of high temperature during vegetative, reproductive and ripening growth phases on its growth, yield and yield components. Considerable variation of cultivars was observed among genotypes under HS and NS environments. Physiological parameters identifying heat tolerant variety is a necessary and is reliable and superior to all the screening procedures. High temperature induces sterility if the sensitive physiological processes are affected. The average grain yield decreased by 22% under HS relative to the NS environment, with high temperature having the greatest effect on spikelets and pollens sterility in New Valley conditions. Higher spikelet sterilities ranging between 14% and 80% were noted under HS than under NS (10% and 18%), while pollens sterility was also higher under HS (15% and 54 %) than under NS (16% and 34 %). Grain yield was negatively correlated with pollen and spikelet sterility under both HS and NS. Potential donors identified based on at least one trait included ARC11094, Pinulupot1, Tupa729, Shinriki, Kameji and Giza 178.