RP-HPLC method has been developed and validated for quantitative determination of Aprepitant in bulk and pharmaceutical dosage forms. All the parameters of the drug met the criteria of ICH guidelines for method validation. The method is very simple, rapid and economic in nature as the peak is well suitable for routine quality control analysis work. Agilent Polaris ODS C18 (150 X 4.6 cm 5μm) column is used. Mobile phase composition consisted of (80:20 v/v) of Methanol and 10mM Sodium acetate (pH adjusted to 3.0 ± 0.1 with glacial acetic acid) on isocratic mode. The flow rate of the method is 1.0 ml/min. The retention time of the drug is 4.40 minutes. Calibration standards were prepared in the concentration range of 2.53 to 50.57μg/ml. The wavelength of detection is 210nm. The column temperature is maintained at 25 0C. Several modifications in the mobile phase composition were made in order to study the possibilities of changing the selectivity of the chromatographic system. These modifications included the change of the type and ratio of the organic modifier, pH, flow rate, temperature and stability of Aprepiitant also studied in various stress conditions.