Plants have developed mutualistic symbiosis with diverse organisms for their mutual benefit. The symbiosis between fungal endophyte Neotyphodium coenophialum and the cool season grasses tall fescue ( Festuca arundinacea Schreb) represent a widespread type of mutualism. Infected plant express increased tolerance to biotic and abiotic stresses, which may result in better persistence. Some of the benefits derive primarily from the production of toxic alkaloids by the endophyte within the host plant. Therefore in the presence of endophyte, greater plant production capability could lead to greater C input to soil. Moreover, residue quality can be changed by the presence of endophyte metabolites (i.e., various alkaloids and phenolics produced by the endophyte association). The (bio)chemical characteristics of plant residues are important factors that governing residue decomposition in soil. Therefore, the objectives of this study were to investigate i) the effects of tall fescue genotype on residues quality, C and N mineralization kinetics and some of the soil biological properties. ii) the effects of endophyte symbiosis on tall fescue residues quality, C and N mineralization kinetics and some of the soil biological properties. For this purpose, soil samples were collected from 0 to 15 cm depth, Chamaseman and Shervedan research stations. Two tall fescue genotypes (B and C) either infected by their natural fungal endophyte N. coenophialum (E+) or their non-infected isolines (E-) were chosen in this experiment. Soil samples were mixed thoroughly with tall fescue plant residues including BE + , BE - , CE + , CE - at the rate of 10 g C kg -1 soil and incubated at 25 °C and 50% WHC. The C mineralization kinetic was monitored during 60 days and some of the biological properties were evaluated after 3 months of incubation. Results indicated that C, cellulose, hemicellulose and lignin concentrations were higher in the residues of E + than E - . In contrast, N concentration was lower in the E+ residues. The cumulative amounts of organic C mineralized in all treatments were non-linear over time. The influence of endophyte on C mineralization was time dependent, so that the decreasing effects of endophytes increased with time. Furthermore, in the both tall fescue genotype net N mineralization-immobilization (hy; m/i ), net changes of amino acid concentration and arginine ammonification were significantly lower in E + treatments. Tall fescue genotype and endophyte status did not affect the enzyme activities in both soils. To study the kinetics of N mineralization and amino acids, plant residues of BE + and BE - were added to Shervedan soil and the soils were incubated for 22 Key Words : symbiosis, endophyte, tall fescue, residue quality, C and N mineralization