Abstract The study was aimed to investigate the antioxidant and organic acid composition of Geaster saccatum mushroom by chemical and electrochemical assay. The antioxidant activity was done by using DPPH and ABTS reagents while the cyclovoltametry (CV) and differential pulse voltamery (DPV) were used to determine its electrochemical behavior at single walled carbon nano tube paste electrode in 0.02 M acetate buffer (pH 5.0), to explore the adsorptive character of its antioxidant compounds at working electrode as a function of pH, scan rate and potential. The organic acid analysis of this mushroom was carried out by High performance liquid chromatography. In DPPH, the scavenging rate was 56.4 ± 0.78%, at 20 µg/ml, reached to 93.4 ± 0.93% at 100 µg/ml, with 61.01 μg/ml as its IC50. Similarly, in ABTS, at same concentrations, scavenging rates were 38.14 ± 0.54% and 95.16 ± 0.43%, with 58.03 μg/ml as its IC50, respectively. HPLC quantified eight organic acids with gallic acid at highest concentration (193.34 mg/kg dry wt) followed by ketoglutaric acid (181.16 mg/kg dry wt.) and oxallic acid (69.54 mg/kg dry wt.). The CV produced a clear irreversible anodic peak at Epa = 0.69 ± 0.01 V, while, ascorbic acid produced a single anodic peak at Epa = 0.29 ± 0.01 V and gallic acid produced two anodic peaks at Epa = 0.41 ± 0.01 V and second at Epa = 0.69 ± 0.01 V. However, only one anodic peak of mushroom was found in DPV at 0.56 ± 0.01 V, which increased proportionally at each increasing concentration of mushroom extract. The results declared this mushroom as one of the best source of organic acids which characterized it as a better antioxidant food as well as medicine.