Determination of optimal plant density and cultivar selection is one of yield increment approaches in grain sorghum. In this in regard, a field experiment with factorial arrangement in randomized complete block design (RCBD) with tree replicates was done at Birjand in 2003. In order to evaluate the effects of plant desity (100000 , 180000 and 260000 plant/ha) on seed yield , yield components, and some physiological traits in 4 grain sorghum varieties (Sepideh, Saravan Local , Payam and Kimia). The results showed that the density increment (from 100000 to 260000 plant/ha) had advantage for seed yield. However, the lower density , due to reduction of plant competition, resulted in a significant increase in single plant yield, number of seeds per panicle , remobilization percentage of different parts of stem and stem remobilzation percentage. The highest stem remobilization percentage belonged to top of stem internode. Effect of density was not-significant on traits such as chlorophyll index and 1000 kernel weight. Cultivars were significantly different in all of studied traits. Intreaction between variety and density was significant on stem remobilization percentage and number of seed per panicle. There was a significant relationship between grain yield , 1000 kernel weight (r = 0.73**) and number of seed per panicle (r = 0.62**). It was concluded that Saravan local variety with grain yield of 12.75 ton/ha and density of 260000 plant/ha were the best combinaton of tratments