Abstract KamLAND-Zen searches for neutrinoless double-beta decay with 136 Xe loaded liquid scintillator (LS). The LS container, a 25- μ m-thick clean nylon inner balloon is a key of the experiment since one of the main backgrounds in the ROI is 214 Bi from the inner balloon. In KamLAND-Zen 400 (operated from 2011 to 2015), dust contamination of the inner balloon from the environment limited the sensitivity, although the inner balloon was fabricated at a class-1 super clean room. We improved the production method of the inner balloon for KamLAND-Zen 800 (started DAQ in January, 2019) and successfully reduced the 214 Bi background level to one fifteenth as compared to the 2nd phase of KamLAND-Zen 400. The inner balloon film material and requirements, improved fabrication scheme, including establishment of clean environment and dust control, will be described.