The main objective of the current work is to develop and validate a new RP-HPLC method for estimation of Bilastine and Montelukast in bulk and in their combined tablets. A good separation of both analytes in various types solutions was attained by using a Ascentis C18 (150 x 4.6mm, 2.6µ) column with a solvent or mobile phase of 0.1% OPA: acetonitrile (50:50 v/v) at a flow rate of 1ml/min and a detection wavelength of 230nm. To test the stability of the analytes, the drug substance was put in an environment with a lot of stress, such as hydrolysis with acid and base, peroxide oxidation, and thermal degradation. At at 2.25min and 2.78 min, Bilastine and Montelukast were eluted with isocratic elution. The method is expected to show a linear response from 2.5 to 15µg/ml for Bilastine and from 5 to 30 µg/ml for Montelukast. Bilastine's LOD and LOQ were establishe to be 0.1and 0.31µg/ml, while Montelukast's were 0.3 and 0.91µg/ml. From the Bilastine and Montelukast peaks, the degradant peaks that were made were easy to tell apart. The method was very sensitive, accurate, cost-effective, and showed stability.