Amorphous sodium silicate (ASL) is found to show high flame retardancy on poly(ethylene-co-vinyl acetate) (EVA), in contrast to crystalline sodium disilicate, Na2Si2O5, with similar composition. The EVA polymer (ethylene content: 67wt%) loaded with 33wt% or more of ASL shows self-extinguishing properties and meets the UL94V-0 rating, while the EVA polymer loaded with 33wt% of the crystalline Na2Si2O5 does not even meet the UL94 HB rating. The flame retardant effect of ASL is much higher than the conventional halogen-free additives such as magnesium hydroxide. The TG measurements of the flame retardant EVA samples and the scanning electron microscope observations of the combustion residues suggest that the remarkable flame retardancy is due to the film-forming properties of ASL.