Silicon nitride (SiNX) and silicon oxide (SiOX) films deposited on flexible polyethersulfone (PES) substrates by plasma-enhanced chemical vapor deposition have been investigated for water vapor transmission rate (WVTR) and oxygen transmission rate (OTR) barrier applications. Details of the NH3/SiH4 and N2O/SiH4 flow ratio effects on the SiNX and SiOX properties in terms of transmittance, refractive index, surface morphologies, OTR and WVTR were investigated. Under optimum conditions, the WVTR and OTR for SiOX/PES can be reduced from a level of 28 g/m(2)/day and 243 cc/m(2)/day (bare substrate) to 0.15g/m(2)/day and 0.25 cc/m(2)/day. Furthermore, the WVTR and OTR for SiNX/PES can be reduced to 0.01 g/m(2)/day and 0.01 cc/m(2)/day. The transparency of the barrier coatings can achieve over 80%. As a result of experiments, the deposition of SiOX and SiNX thin films on PES plastic substrates can resist the moisture and oxygen transmission sufficiently.