Fabrications of barrier layer on a polyethersulfon (PES) film and OLED based on a plastic substrate by atomic layer deposition (ALD) have been carried out. Simultaneous deposition of 30 nm of $AlO_x$ film on both sides of PES film gave film MOCON value of 0.0615 g/$m^2$/day (@38$^{circ}C$, 100 % R.H.). Moreover, the double layer of 200 urn $SiN_x$ film deposited by PECVD and 20 nm of $AlO_x$ film by ALD resulted in the MOCON value lower than the detection limit of MOCON. The OLED encapsulation performance of the double layer have been investigated using the OLED structure of ITO/MTDATA(20 nm)/NPD(40 nm)/AlQ(60 nm)/LiF(1 nm)/Al(75 nm) based on the plastic substrate. Preliminary life time to 91 % of initial luminance (1300 cd/$m^2$) was 260 hours for the OLED encapsulated with 100 nm of PECVD deposited $SiN_x$/30 nm of ALD deposited $AlO_x$.