This investigation was carried out to study the effect of temperature resulting from different sowing dates during the stage of flowering and seed maturity as well as the effect of genotypes x environment interaction on seed yield and its components in some populations of alfalfa (Medicago sativa L.). A set of six populations from Egypt and one Genan from U.S.A. (Aswan, Balady, Dakha, Ismailia-1, Nitrogen fixed and Siwa) were sown on three sowing dates i.e. 20^th of each of October, November and December in randomized complete block design with three replicates in the experimental farm of South Valley University during 2014/2015 and 2015/2016 seasons. Three cuts were taken after 80, 125 and 165 days from sowing. The results revealed that the highest number of pods/plant (63.8), 1000 seed weight (2.77 g) and seed yield/plant (0.86 g) were obtained from sowing on the 20^th November. This result is due to that seed production is taking place during the April month. The Aswan population outyielded (134.8 g/m²) other tested populations. On the other hand, Genan U.S.A. population gave the lowest yield (51.2 g/m²) over all sowing dates and seasons. Moreover, the environments, populations and their interaction were signficiant for number of pods/plant, number of seeds/pod, 1000 seed weight and seed yield. The estimates of genotypic stability parameters (a and l) for seed yield and its components showed that the average stability region involved Ismailia-1 population for number or pods/plant; Balady, Ismailia-1 and Nitrogen fixed populations for number of seeds/pod; Ismailia-1 for 1000 seed weight and Nitrogen fixed for seed yield/m². Also, the results showed that Aswan population was the highest seed yielding, but exhibited less stability (instable in seed yield). In contrast, Nitrogen fixed population showed lower seed yield, but in stable region