This paper demonstrates a micro-machined ultra-sensitive bi-directional magnetic sensor based on the Lorentz-force principle. The magnetometer relies on detecting the resonant frequency of electrothermally actuated straight micro-resonator operated near the buckling point. Optical sensing is utilized to capture the frequency shift while the device is operated at ambient pressure and temperature. The micro-sensor shows a high normalized sensitivity (S) of 9.48 (mA.T)-1 compared to the literature and good linearity of 0.12% in wide range of magnetic field. Also the sensor consumes low power around 0.2 mW. These features encourage for low cost applications.