Using 100% industrial fabrication processes, the design and the fabrication of a monolithic CMOS MEMS magnetic field sensor, targeting noise reduction, are developed in this paper. The sensor is based on a resonant cantilever structure with optimized electronics for signal treatment and noise filtering in order to achieve competitive performances. With a final sensitivity of 350Vrms/T and a 2μT resolution, the system is able to measure earth natural magnetic field.