The multiple cracking progress in thin SiOx films deposited on polyethylene terephthalate (PET) substrates during tensile tests were investigated. Five kinds of specimens with different SiOx film thicknesses of 43, 67, 90, 120 and 320 nm were prepared. With the appropriate estimation of the residual strain in the film, the crack onset stress was found to be nearly proportional to the minus one-half power of the thickness. After the cracking is sufficiently developed, on the other hand, thinner specimens showed higher crack density than thicker ones. In order to predict the multiple cracking progress, stress distributions in the cracked films were calculated both by the elastic-plastic finite element analysis and modified shear lag analysis. The prediction, under the assumption of the unique stress criterion, explained reasonably well the multiple cracking progress obtained in the experiments.