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A disposable screen-printed carbon electrode (SPCE) functionalized reduced graphene oxide (rGO) was successfully fabricated for electrochemical detection of stress biomarker cortisol. Firstly, graphene oxide (GO) suspension was drop-casted onto carbon electrode surface before being electrochemically reduced into rGO-modified SPCE. Secondly, the rGO-modified SPCE was treated with N-hydroxysuccinimide (NHS) and N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide (EDC) solution to activate carboxylic group (COOH) on the rGO surface which then reacts with amines (NH3) group of cortisol specific monoclonal antibody (C-Mab) to form an amide bonding. Finally, the electrochemical response of C-Mab/rGO-modified electrode towards cortisol was recorded by cyclic voltammetry (CV) techniques using5 mM (Fe3[CN]6) as a redox probe. The significant change of current was observed with and without the presence of cortisol using our fabricated electrode, C-Mab/rGO-modified SPCE becomes the main principle in this cortisol electrochemical immunosensor. The incorporation of rGO onto SPCE surface exhibited an enhancement of electrocatalytic of electrochemical oxidation of Fe3[CN]6for the detection of cortisol. It was shown that the peaks current of Fe3[CN]6 was significantly reduced in the presence of cortisol onto the C-Mab/rGO-modified electrode surface. Several parameters were optimised for improving electrochemical detection of cortisol including rGO concentration, C-Mab concentration and immobilisation time of cortisol. Under optimal condition, our developed immunosensor was able to detect cortisol in a wide linear range of 0.001 µg/mL to 10 µg/mL which covers the human cortisol levels in normal and stressful environments. These findings would help in the development of biosensor to monitor the stress level among military personnel. [ABSTRACT FROM AUTHOR]