The sensitivity of the magnetoresistive sensors based on magnetic tunnel junctions (MTJs) are to reduce sources of noise, to increase the signal, and to understand the fundamental limitations involved. Relevant properties, such as tunneling magnetoresistance ratio (TMR), coercivity (Hc), exchange coupling field (He), domain structures, and noise sensitively depend on the free layer structure in the MTJ [1-9]. There are also external factors that affect the performance of the sensor, such as the design of magnetic concentrators . We have studied the using of micro-magnetic concentrators and external magnetic concentrators to improve the sensitivity of the magnetoresistive sensor. By optimizing the TMR ratio, Hc, He, domain structures, noise and magnetic concentrators, we fabricate a sensor with a sensitivity as high as 5,146 %/mT. This magnetic sensor only dissipates 200 μW of power while operating under an applied voltage of 1 V.