In this research, we obtained a single chip sensor for weak current measurement. The working principle of this sensor is utilizing the TMR device as magnetic field sensing unit, and integrating current conductors to reduce the distance between the current conductors and TMR device, and therefore, to improve the resolution in current measurement. We fabricated a weak current sensor with the resolution of 20 μA using MEMS process by optimizing the structure of both TMR device as well as current conductors. This current sensor offers a promising and economic solution for weak current measurement.