In order to put organic light emitting diodes (OLEDs) into practical use, high barrier films with 10－6 g m－2 day－1 level of water vapor transmission rate (WVTR) are required. Allthough various techniques for evaluating high gas barrier properties have been proposed, WVTR measurement methods with the detection limit of 10－6 g m－2 day－1 or less have not been developed so far. The author succeeded to develop a WVTR measurement unit to measure 10－6 g m－2 day－1 or less by applying ultra-high vacuum technologies. The measurement result of two gas barrier films with 10－6 g m－2 day－1 and 0.1 cc m－2 day－1 level of oxygen transmission rate (OTR), respectively, are shown.
