SummaryTo study the effects of high temperature on the meiosis of pollen mother cells (PMCs) and to determine their relationship with grain set, two in situ experiments were conducted between 2010 and 2012 on four wheat cultivars, Kauz, Montana, M6 and Chamran, under normal (normal cultivation) and terminal heat stress (late cultivation) conditions. Due to the delay in cultivation for the cultivars under stress conditions, their flowering stage faced heat stress at the end of the growing season, and therefore, a significant (p ≤ 0.05) increase was observed in meiotic abnormalities. Cytogenetic studies discovered that the meiosis in PMCs and the pollen development are strongly influenced by heat. Based on the results, abnormalities such as precocious chromosome migration to the poles, laggard chromosomes, micronuclei, absence of metaphase plate, pyknosis, abnormal cytokines, cytomixis and abnormal tetrad were observed in different cultivars. The sensitive cultivars, M6 and Montana, had the greatest percentages of meiotic abnormalities. The meiotic abnormalities showed a significant negative correlation with the number of kernels per spike under terminal heat stress conditions (r = 0.54, n =16, p ≤ 0.01). In other words, due to increasing meiosis abnormalities under terminal heat stress, the grain set was greatly reduced, especially in the susceptible cultivars. The result of this experiment showed that studying the meiosis in PMCs can be suggested as one of the research necessary to improve commercial heat-tolerant cultivars, and to some extent, as a method for screening tolerant lines in breeding programs.