In the food packaging industry, transparent gas barrier films consisting of a 10-100 nm thick glass layer deposited on a polymer substrate are being developed. The accurate densitymeasurement. of the thin glass layers with thickness ranging from 10 to 100 nm was important to characterize nanostructures, hence the gas permeation property of the films. First, densities of SiOx layers which were deposited by vacuum evaporation on silicon wafer surfaces(100) were measured with a weight-volumetric method. The densities were 2.15+/-0.01 andi 2.16+/-0.03 g/cm(3) for layers with slightly different deposition conditions. Next, a novel method was invented for the density measurement of thin SiOx layers deposited by vacuum evaporation on polymer substrates. The method made use of the fact that parallel multiple cracks were induced in the SiOx layer when the film was' stretched. The thickness of the SiOx layer was estimated by measuring the depths of crack openings. The density was found to be larger: 2.32+/-0.09 g/cm(3).