Several authors have investigated the role of plant breeding in increasing the wheat grain-yield potential and its associated physiological changes. Increasing the understanding the physiological changes may allow the development of new selection criteria for improving grain yield. One method to identify which crop characteristics would be more useful in future breeding programs is to study old and modern cultivars and to determine which yield components are simultaneously changed with grain yield. Therefore, studies were carried out, during three years, at the National Wheat Research Center, in Passo Fundo-RS, Brazil (28^o15’S,52^o24’W and 687 m above sea level) where seven wheat varieties, which covered reasonable cropping area during a period of 52 years (Frontana , 1940; BH 1146, 1955; Nobre, 1969; IAS-54, 1970; CNT-8, 1976;Trigo BR 23, 1987 and EMBRAPA 16, 1992), were compared and some physiological aspects associated with grain yield were examined. Grain yield, biological yield, biomass partitioning, harvest index and grain yield components were also determined. Grain yield changes during the period of study were more associated with biomass production (r=0.78 p<0.01) than with the harvest index (r=0.65 p<0.01). Biomass partitioning to the spike at anthesis and spike dry weigth at anthesis were significantly and positively correlated with grain per square meter and harvest index. The genetic improvement of wheat has positively affected the grain yield by changes in duration of the spike growth period and biomass partitioning to reproductive organs in preanthesis stages. In relation to yield components, the number of grains per square meter was more affected by plant breeding and was more correlated with grain yield (r=0.94 p<0.01) than with grain weight (r= - 0.39 n.s.). Also, the high number of grains per square meter in the modern cultivars was due to the production of a high number of grains per spike.