We proposes a platform which can generate hardware/software description based on flexible in-struction set architectures (ISAs). The platform takes advantage of the flexibility of field pro-grammable gate array (FPGA) to design many micro control units (MCUs) based on different ISAs. The platform can generate many ISAs, MCUs, and Assemblers according to a pre-defined ISA and user applications. Although the MCU performance is not optimized, the FPGA shows a great potential on resource reduction and enough performance at very low system clock rate. The flexible ISA has shown great importance for the design targeted to specific purpose. We also show a case study of the proposed flexible ISA-based FPGA-MCU. It can control many specifi-cally designed hardware IPs and a customized multi-task OS with tasks. Not only the case works correctly, but also the proposed FPGA-MCU of the case is flexible with reduced FPGA resources, low cost, and within time constraints.