Rice (Oryza sativa L.) is the one of the most important cereal crop and staple food of over half the world’s population that provides 45-60% of the dietary calories . The global climate changes including increased heat affect negatively rice production and other crops resulting into increased food insecurity. The analysis of Induced gamma rays mutations from upland rice mutant lines was done to discover mutations in heat tolerant genes (HSPs genes). Out of 64 putative heat HT mutant upland rice lines characterized for mutations in HSPs genes, 34 lines discovered to have mutations in that gene by using TILLING (Targeting Induced Local Lesions IN Genomes) technique with SNP Markers; gene specific primers. The results of nucleotides sequenced of mutant rice lines DNA, indicated that most mutations discovered were base pair substitution and InDels 50% and 41% in OS_HSP90_1 and 23% and 35% in OS_HSP17.9 respectively. Mutant rice lines identified to have HSPs genes were evaluated for growth performance, yield and yield components in order to select the promising HT mutant rice lines which can produce economic yield under heat and drought stress conditions. The 8 Mutant rice lines produced economical yield under heat stress condition selected for further breeding as donor materials for heat tolerant. This study aimed to determine the genetic factors associated with heat tolerance in mutant upland rice lines for variety development by Marker Assisted Selection (MAS). Understanding genetic mechanisms for heat stress tolerance in mutant rice lines will help further breeding and selection of suitable rice breeding materials for heat tolerance in order to improve rice productivity hence to reduce food insecurity.Tanzania Official Seed Certification Institute (TOSCI)