Objective: Aim of this study was to develop and validate the RP-HPLC method using Box-Behnken Design (BBD) for simultaneous analysis metformin HCl and glimepiride in spiked plasma. Methods: The chromatographic system was comprised of acetonitrile-phosphate buffer 0.0125 M+Sodium Dodecyl Sulphate (SDS) 1 mmol as a mobile phase and Ascentis® Phenyl C18 (250 x 4.6 mm i.d.; 5 µm) column as a stationary phase with UV detector at 210 nm. Three independent variables included phosphate buffer (%), pH and flow rate were optimized using Box-Behnken Design. The observed responses were retention time, peak area and resolution. Results: The predicted optimum condition of the RP-HPLC system consisted of phosphate buffer solution of 72%, pH at 4.3 and flow rate at 0.8 ml/min. By using this condition, the duration of analysis was more than 18 min, so it was necessary to modify the flow rate to be 1.0 ml/min to get shorter analysis duration. This condition was then applied to analyze metformin and glimepiride in spiked plasma and validated according to the EMA guideline. AUC of interfering components at the IS retention time between 588-1092 mV, the linearity of metformin was 0.9993 and glimepiride was 0.9991, accuracy and precision were between-13.33% until 16.08%, dilution integrity and metformin stability studies were between-4.01% until 11.82%, and for glimepiride stability studies were between-37.48% until-4.76%. Conclusion: Box-Behnken Design can help optimize the HPLC system, and the optimum condition was valid to analyze metformin and glimepiride in spiked plasma by considering the storage time of plasma samples.