Traffic stress occurs across all ornamental and sport lawns. Traffic means lawn damages as a result of compaction, tearing andetc. due to movement of players, machineries and animals. Studies have shown that theapplication of plant growth regulators is one of the ways for improving thetraffic resistance. Trinexapac-ethyl is a plant development retarder widely used in lawn management. Although there are some evidence that trinexapac-ethyl may improve lawn Traffic resistance,but there are rare data on trinexapac-ethyl resistance mechanism.The main objective of the present studywas to investigate the effect ofTrinexapac-ethyl on Traffic stress resistance of two cold season lawn species and characterization Trinexapac-ethyl and traffic impacts on the morphological and physiological traits of these species. The present research was conducted in 2011-2012 in Esfahan University of technology within landsaround greenhouse of agriculture science department on two lawn species of Wheatgrass and Tall fescue. The experiment was arranged as factorial in completely randomized block design with three replicates. Treatments involved two species, three levels of Trinexapac-ethyl and two Traffic levels.Trinexapac-ethyl was applied in three levels (0, 0.25 and 0.5 kg/h) on plots in dimension six m2 in three times and three weeks interval. ArtificialTraffic was applied on half on plots by simulator device. Results showed that Trinexapac-ethyl and Traffic decreased plant height,the shoots fresh and dry weight. The levels of 0.25 and 0.5 kg/h reduced Wheatgrass and Tall fescue height about `18.93, 27.02, 12.08 and 15.01 % respectively. In both lawn species, Trinexapac-ethyl increased color intensity and in contrast, traffic stress caused plant b to be paled. Wheatgrass density was improved and decreased under Trinexapac-ethyl and Traffic stresses respectively. Both treatment did not affect Tall fescue significantly0.5% . Trinexapac-ethyl improved tillering in Wheatgrass and Tall fescue about 31.14 and 17.87% respectively. Traffic didnot affected both species tillering significantly. Results of research showed that Traffic resulted in low leaf relative water content, chlorophyll content, antioxidant activity and higher electrolyte leakage in both species. The plants treated with Trinexapac-ethyl during experiment showed higher relative water content than control ones, also electrolyte leakage for both species was reduced significantly by Trinexapac-ethyl. At the same time, Trinexapac-ethyl accelerated enzymes activity in scorbat peroxidase, peroxidase and super oxide desimotase under Traffic stress.However catalase activity did not affected by bothTrinexapac-ethy land stress significantly. Finally Trinexapac-ethylimproved Trafficresistancethrough enhancing osmotic regulation and enzymeactivity in both Wheatgrass and Tall fescue. Additional index words : Trinexapac-ethyl, Tall fescue, Wheatgrass, Traffic stress, Antioxidant enzymes, Growth rate