Cat's whiskers (Orthosiphon stamineus) is a medicinal plant which comprises several dynamic pharmacological properties such as anti-inflammatory, antioxidant and antibacterial. In small and medium-scale industries, conventional reflux extraction method is favored as compared to other non-conventional extraction methods due to cost effectiveness and simple operating procedures. In this study, response surface methodology (RSM) was applied to optimize the reflux conditions for extraction of cat's whiskers leaves in order to achieve a high content of antioxidant activity in the extracts. Central composite experimental design (CCD) with three factors and three levels was employed to consider the effects of the operation conditions. Antioxidant activity of the extracts were based on free radical scavenging activity (DPPH assay) and were analyzed using a UV-Vis spectrophotometer. Based on RSM, the antioxidant activity could be maximized when the operation conditions were 125 µm for particle size, 1.5:20 for sample-to-solvent ratio, and 2 h for extraction time. Under these optimal conditions, the predicted value of the antioxidant activity was compared with the actual, and the mean error was 0.46%. This indicates the suitability of the model for optimizing the conditions for the reflux extraction of cat's whiskers leaves.