Seed filling in wheat is controlled by the availability of substrate (source strength), the capacity of the organs to utilize it for seed growth (sink strength). The aim of this research was to study source-sink relations in wheat, through seed weight and grain number in new and old wheat cultivars. The experiment was performed at Moghan College of Agriculture and Natural Resources, Research Farm (University of Mohaghegh Ardabili) during 2010-2011 growing season. The plant materials (81 wheat cultivars) were evaluated using a simple lattice design with two replications under well-watered condition. The results showed that there were significant differences for measured agronomic and physiologic traits among wheat cultivars tested. Overall, new wheat cultivars showed high values of seed yield as compared with the old ones. This trend was also observed in the cases of harvest index (HI) and grain number per square meter. There were no significant differences in biological yield and 1000-grain weight between old and new cultivars. Spike dry weight measured at anthesis and physiologic maturity and also partitioning photoassimilates to the spike during anthesis-physiologic maturity phases increased more in new cultivars than old ones. Increased grain number in new wheat cultivars and the lack of significant changes in 1000-seed weight suggests that seed yields of the cultivars used under the condition tested are controlled more by sink than source strength. Under conditions tested, it seems that increased grain number, either in plant level or in square meter, might be considered as a way to assess increased seed yield.