The present invention relates to a method of forming an insulating film in a semiconductor device. After a mixed gas of alkyl silane gas and N.sub.2O gas is supplied into the deposition equipment, a radio frequency power including a short pulse wave for causing incomplete reaction upon a gas phase reaction is applied to generate nano particle. The nano particle is then reacted to oxygen radical to form the insulating film including a plurality of nano voids. A low-dielectric insulating film that can be applied to the nano technology even in the existing LECVD equipment is formed.