The passivation layers that should be formed on flexible electronics devices require high mechanical stability. Therefore, organic passivation has been used to enhance the electrical characteristics of various devices such as thin-film transistors (TFTs), organic light emitting diodes, and capacitors under mechanical stress. However, the conventional deposition of an organic film based on spin coating results in excessive thickness and the potential for chemical damage due to by-products such as organic solvents throughout the process. Here, we present the effects of a poly(1,3,5-trimethyl-1,3,5-trivinyl cyclotrisiloxane) organic passivation layer deposited by initiative chemical vapor deposition (iCVD) on the electrical and mechanical stabilities of oxide TFTs subjected to 30 000 repetitive tensile bending cycles. The highly ultrathin (50 nm) and excellent roughness (Rq = 0.33 nm) of the pV3D3 film assisted in preserving the mechanical stability of the device under external mechanical stress, and degradation of the electrical properties was suppressed compared with a device using SiO2 passivation. The mechanical properties of the type of passivation layer, including its Young's modulus, affected the degradation of the electrical properties and reliability characteristics under repetitive bending. Finite-element structural simulations indicated a 15% reduction in equivalent stress applied to each layer of the device when pV3D3 (versus SiO2 passivation) was used. The iCVD-deposited pV3D3 film used in this study is a powerful candidate to act as the passivation layer of flexible electronics by strengthening the electrical stability of a device under external mechanical stress.& nbsp;Published under an exclusive license by the AVS.