In this paper, we demonstrate an ACF-packaged ultrathin Si-based flexible NAND flash memory by adopting a simple method, without using a conventional transfer process. By gently etching the bottom sacrificial silicon of the SOI wafer, flip-chip bonded devices were sufficiently thinned down (roughly to 1 μm) to fabricate highly flexible, fully packaged Si-based NAND flash memory, without any cracks or wrinkles. The work presented here suggests a useful methodology to realize various high-performance, fully packaged Si-based flexible LSI devices.