A nozzle assembly of a vacuum cleaner is disclosed. The A nozzle assembly includes a main casing having an air inlet formed thereto to draw in dust or dirt from a surface to be cleaned, a drum brush disposed in the main casing to brush away the dust or dirt adhered to the surface while rotating in contact with the surface, and a subsidiary brush unit disposed in front of the main casing to preliminarily brush away the dust or dirt adhered to the surface in front of the drum brush, and having a cover disposed in front of the main casing, and a subsidiary brush member to come in elastic contact with the surface. The subsidiary brush member is configured to change a relative position to the cover, and thus to selectively maintain a contact state or a non-contact state to the surface.