The multiple cracking phenomena in thin SiOx films deposited on 12 mu m-thick polyethylene terephthalate (PET) substrates during the tensile test are investigated. Thicknesses of SiOx films ranged from 43 to 320 nm. The multiple cracking progress is observed in situ by optical microscopy and from which the crack density in SiOx films is measured. The predicted crack density by the shear lag analysis including residual strains, explains reasonably well the experimental results. The critical energy release rate, G(c), for the first film cracking is also evaluated from simple energy balance arguments. Although it depends on the analytical model, G(c) is estimated to be a constant value of about 1.0 J/m(2) regardless of the thickness.