The genetic system controlling variability in stem diameter and number of vascular bundles in wheat was investigated in an 8 parent diallel cross  grown in favorable environment (normal sowing date) and heat stress environment (late sowing date). For the two stem attributes genes with additive effects were operating with non-allelic gene interaction being involved for number of vascular bundles under normal sowing date, heat stress reduced stem diameter by 14.1% on average while the average reduction in vascular bundles amounted to 10.65%. The narrow-sense heritability estimates were reasonably. high and comparable in the two environments being 0.78and 0.62 for stem diameter, 0.78 and 0.73 for number of vascular bundles under favorable and heat stress conditions, respectively. The two stem attributes were positively and significantly correlated in both favorable (r = 0.77, p < 0.01) and heat stress environment (r = 0.69, p < 0.01), indicating the considerable proportion of the variation in stem diameter is accounted for by variation number of bundles in the stem. The impact of heat stress was grater on grain yield per spike (38.5% reduction) than on 1000 kernel weight (22.1% reduction). For the two yield components analyzed, the narrow-sense heritability was rather low being 0.47 and 0.40 for grain yield per spike in normal and stress environments. Stem diameter was significantly correlated with both; 1000 kernel weight under favorable (r = 0.59, p < 0.01) and heat stress condition (r = 0.59, p < 0.01). Stem diameter was also correlated with grain yield per spike in the two environments (r = 0.47, and 0.42, p < 0.01). Meanwhile, number of vascular bundles was significantly correlated with 1000 kernel weight under favorable conditions only (r = 0.53, p < 0.01) and with grain yield per spike on the two environments (r = 0.47 and r = 42, p < 0.01). Moreover, the positive significant association between stem diameter and 1000 kernel weight was consistently displayed in the 9 F₂ segregating populations analyzed. Meanwhile the association of stem diameter with grain yield per spike was significantly positive in only six of the 9 F₂ populations. The results of the present study clearly demonstrate the utility of selecting for stem diameter for improving heat stress tolerance as an easy storable character with reasonable high narrow-sense heritability which will increase storage ability of assimilates in the stem via affecting number of vascular bundles.