A nozzle assembly of a vacuum cleaner is disclosed. The nozzle assembly includes a nozzle assembly body having an air inlet and an air outlet, a drum brush unit disposed in the nozzle assembly body in the vicinity of the air inlet, and having a drum brush disposed to brush away dirt or dust adhered to a surface to be cleaned while coming in rotation contact therewith, and a movable brush unit disposed in at least one of the front and the rear of the air inlet on an undersurface of the nozzle assembly body to pivot in an opposite direction to a moving direction of the nozzle assembly body thus to change a contacting angle to the surface to be cleaned in the range of a predetermined angle and then to brush away dirt or dust adhered to the surface to be cleaned, in moving of the nozzle assembly body.