The present analytical work is a unique method development and validation for the simultaneous determination of Atazanavir and Cobicistat by using reverse phase ultra-performance liquid chromatography (UPLC) with isocratic elution technique. Here the stationary phase used was C18 HSS column (2.1 × 100 mm, 1.8 μm) mobile phase was 45% OPA (0.1%) and 55% Acetonitrile. pH of the mobile phase was maintained at 3.0, flow rate 0.2 ml/minute. Eluted material underwent for monitoring at the detector wavelength of 254 nm. Retention time for Atazanavir and Cobicistat was found to be 0.536 minutes and 1.366 minutes, linearity range was 75 μg/ml to 450 μg/ml and 37.5 μg/ml to 225 μg/ml respectively. The new method was evaluated according to ICH guideline and as far as validation results are concern correlation coefficient value was 0.999 for both of the compounds, LOD 0.76 and 0.37, LOQ 0.2.30 and 1.11, percentage recovery 99.74% and 99.34%, repeatability results relative standard deviation (%RSD) 0.4 and 0.6 for Atazanavir and Cobicistat respectively. The developed UPLC method was found to be a simple and rapid one for regular analysis in professional laboratory.