Forage and pasture plants comprise worth while values economically and allocate a considerable part of the agricultural trades. Tall fescue ( Schedonorus arundinaceus Schreb.) is one of the most important, perennial and cool-season grasses that has the C3 photosynthesis system. This plant grows in different parts of Iran. Tall fescue is naturally open pollinator and has a gametophytic self-incompatibility system . Some studies have shown the production of self-pollination derived seeds in symbiotic populations with Epichloe endophytic fungi. Several studies have been also conducted on the positive effects of endophytic fungi presence on the characteristics of tall fescue. The present study was done to evaluate forage yield, morphological and phytochemical traits in self-pollination derived populations containing endophyte and self-pollination effects on this symbiosis. This study was done during 2016-2017 in the research farm of Isfahan University of Technology located at Lavark, Isfahan. The plant material used in this experiment consisted of two self-pollinated Iranian populations infected with endophytic fungi (E+), 10 hybrid populations and 7 self-pollinated forein populations as control. The experiment was executed in a randomized complete block design with 3 replications. The results of analysis of variance for morphological traits showed population as a source of change for all traits. This indicates that the genotype of the plant has a significant effect on these traits. Comparison of means for height, leaf diameter and area traits showed that the control group had lower mean scores than their hybrid populations. While the self-pollination derived populations containing endophyte had higher means than their hybrid populations. The comparison of means for wet weight showed that this trait is higher in endophyte-free hybrid populations with a large difference compared to self-pollinated derived populations with endophyte. This is while the difference in dry weight was very low. This clears that endophyte has a positive effect on dry matter content and in fact dry matter yield. Calculating the phenotypic variance of populations for morphological traits showed that endophyte presence reduced this variance among self-pollinated derived populations. Decreasing in variance in two self-pollinated derived populations containing endophytes was different, which could be due to the interaction between endophytic fungi and host plant genotype. The results of phytochemical experiments showed that the content of phenolic and flavonoids in endophyte-infected self-pollinated derived populations was significantly higher than endophyte-free hybrid populations and control populations. The results of high performance liquid chromatography (HPLC) experiment identified 5 main compounds in the samples. These compositions included rutine, P-comaric Acid, chlorogenic acid, caffeic acid and gallic acid. The amount of these compounds in endophyte-infected samples was higher than the endophyte-free ones, with a highly significant difference in rutine composition. Biplot results showed that there is a positive correlation between morphological traits, especially between wet weight and dry weight with each other and height, diameter and leaf area. There was also a positive correlation between phenolic compounds and flavonoids. Biplot results illustrated that endophyte-infected self-pollinated derived populations had optimal performance regarding morphological and phytochemical traits. In this way, self-pollination in the presence of endophytic fungi can lead to the optimal yield and has a positive effect on symbiotic relationship between endophytic fungus and host plant. Thus, the production of pure lines from endophyte-infected self-pollinated derived population in tall fescue plants is recommended. Keywords : Tall fescue, Endophyte fungus, self-pollination, phenotypic variance, phenolic compounds