In this work, we develop novel file system, FRASH, for byte-addressable NVRAM (FRAM[1]) and NAND Flash device. Byte addressable NVRAM and NAND Flash is typified by the DRAM-like fast access latency and high storage density, respectively. Hierarchical storage architecture which consists of byte-addressable NVRAM and NAND Flash device can bring synergy and can greatly enhance the efficiency of file system in various aspects. Unfortunately, current state of art file system for Flash device is not designed for byte-addressable NVRAM with DRAM-like access latency. FRASH file system (File System for FRAM an NAND Flash) aims at exploiting physical characteristics of FRAM and NAND Flash device. It effectively resolves long mount time issue which has long been problem in legacy LFS style NAND Flash file system. In FRASH file system, NVRAM is mapped into memory address space and contains file system metadata and file metadata information. Consistency between metadata in NVRAM and data in NAND Flash is maintained via transaction. In hardware aspect, we successfully developed hierarchical storage architecture. We used 8 MByte FRAM which is the largest chip allowed by current state of art technology. We compare the performance of FRASH with legacy Its-style file system for NAND Flash. FRASH file system achieves x5 improvement in file system mount latency.