The objective of the present study was to determine antioxidant capacity and oxidative stress index of Lepista nuda (Bull.) Cooke mushrooms collected in Şahinbey (Gaziantep/Turkey) using novel research kits that could conduct high sensitive and reliable measurements with in vitro analysis methods. Its antioxidant activity was determined by DPPH method and TAS (total antioxidant status), TOS (total oxidant status) and OSI (oxidative stress index) values were determined using Rel Assay kits. Study findings determined that the TAS value was 3.102 mmol/L, the TOS value was 36.920 μmol/L and the OSI value was 1.190 for the mushroom. It was observed that the radical scavenging activity of DPPH increased in higher concentrations. It was concluded that L. nuda samples collected in regions with low TOS values could be used as better antioxidant sources.