The GRAPES-3 is a high-density extensive air shower (EAS) array located at Ooty India. The array is presently operating with 400 plastic scintillation detectors (1 m^2 area each) with the inter-detector separation of 8 m spread over 25,000 m^2 and a large area (560 m^2) tracking muon detector. The variation in atmospheric pressure can significantly affect the observed cosmic ray flux measured by these detectors. We have simulated this effect by using the CORSIKA package after folding the detector response using GEANT4. The results of this work will be discussed during the conference.