We studied the passivation of pentacene organic thin-film transistors (OTFTs) on plastic using four kinds of organic layers: photopatternable acryl (PA), polyvinylalcohol (PVA)/dichromated polyvinylalcohol (D-PVA), PVA/D-PVA/PA, and parylene-C. The initial mobility of the OTFTs was similar to 1.0 cm(2)/V s and the on/off current ratio was similar to 10(7). The drop in the on-current and the increase in the off-currents were significant when PA was used as a passivation layer because of the damage to the pentacene active layer by the organic solvent in PA solution. On the other hand, there was little change in the performance of the OTFT when PVA/D-PVA was used as the passivation layers on the pentacene. This was done to reduce the concentration of metal ionic impurity at the interface of the pentacene and PVA. The OTFT with the passivation layers exhibited a field-effect mobility of 0.8 cm(2)/V s and on/off current ratio of 10(7). This means that the water soluble PVA can minimize the damage to the pentacene and prevent the penetration of the organic solvent in PA solution.