This invention relates to a process for preparing a pharmaceutical composition comprising four antitubercular drugs: rifampin or a pharmaceutically acceptable salt thereof, isoniazid or a pharmaceutically acceptable salt thereof, pyrazinamide or a pharmaceutically acceptable salt thereof and ethambutol or a pharmaceutically acceptable salt thereof, wherein rifampin and isoniazid are in separate layers. The invention also provides a pharmaceutical composition prepared therefrom having advantageous stability and bioavailability.