To determine the effects of priming and planting density on improvement of morphological and physiological traits of the two hybrid corns SC (S.C. 704) under double cropping in Sari, a split plot experiment as randomized complete block design with four replications was conducted in 2010. Treatments included two densities of 80 and 100 thousand plants per hectare as the first factor and osmopriming of seeds of two hybrids of corn (S.C. 704), with a solution of 10% polyethylene glycol (6000 PEG), 0.5% potassium nitrate (KNO3), 2% potassium chloride (KCl), water (hydropriming) and control (without priming) were considered as the second factor. The results showed that plant height using the priming solution of 10% polyethylene glycol and 100 thousand plants per hectare density was 33 percent higher than 0.5% potassium nitrate and 2% potassium chloride with the density of 80 thousand plants per ha. Also the forage yield per hectare with the density of 100 thousand plants was 14.42% more than 80 thousand density. Best plant density and priming concentration for leaf area index (LAI), crop growth rate (CGR) and net assimilation rate (NAR) were the density of 80 thousand plants per hectare and 10% polyethylene glycol. Therefore, it is concluded that adequate forage yield (54730 kg/ha) of the hybrid corn used in this study were obtained by the density of 100 thousand plants per hectare and 10% polyethylene glycol solution.