films of a plastic resin such as a polyester film for packaging use can be imparted with improved gas barrier property or decreased permeability to gases such as water vapor and oxygen without decreasing pliability or flexibility and without degradation of the appearance and transparency so as to be useful as a packaging film for products having sensitivity to those gases . according to the invention , a plastic film is provided on at least one surface with a vapor - deposited layer of amorphous silicon of 10 to 200 nm thickness . the vapor deposition process is carried out preferably by a chemical vapor deposition method using silane and hydrogen as the reactant gases the gas permeability can be further decreased by a heat treatment of the amorphous silicon layer at 70 to 140 ° c .