A method for forming a high density dielectric film by chemical vapor deposition. The method comprises: (a) a substrate is provided in a processing chamber; (b) a first gas is introduced into the processing chamber with a first pressure and adsorbed on the substrate, wherein the first gas comprises silicon-containing or carbon-containing gas; (c) the first gas is stopped, and the first pressure is lowered to a second pressure; (d) a second gas is introduced into the processing chamber with a third pressure, and forced to react with the first gas absorbed on the substrate and remained in the processing chamber, wherein the second gas comprises oxidizer or reduction agent; (e) the steps (b).about.(d) are repeated until a high density dielectric film is formed on the substrate.