This paper reports the design, fabrication and measurement of a flexible microelectrode array for monitoring the regenerating state of peripheral nerves. Polyimide was used so that the device would have biocompatibility, long-term stability and flexibility. Using a ‘stress-free etch-release’ scheme, it was possible to fabricate a microelectrode array with a smaller feature size because less geometrical deviation took place compared to that using a mechanical peeling-off method. The diameter of the via holes, which provide a path for regenerating nerve fibers, was 40 m and 50 m. In vivo tests were performed by implanting fabricated microelectrode arrays into the sciatic nerves of a rat and into the peroneal nerves of a rabbit using two different implantation methods. One is a direct inter- fascicular suture technique and the other is a method using a nerve conduit. In both cases, no side effects such as cytotoxicity, neuroma and antigen-antibody reaction were observed. In vitro measurement was carried out to confirm the feasibility of measuring nerve signals. The interfacial impedance between body fluids and microelectrodes was measured. The normalized interfacial impedance was 446 /m2 at 1 kHz. Signal transfer characteristics were analyzed by applying square waves of various frequencies and magnitudes.