Aimed at the problem that oil atomic emission spectrometer has low detection efficiency on the particles bigger than 10 μm,a detector of oil ferromagnetic particles (DOFP) was designed based on giant magneto resistive magnetic field sensor.The DOFP has well repeatability on the same oil sample.For the oil samples whose concentration and size distribution of ferromagnetic particles both are different，the detection results have no significant difference due to the difference of size distribution.Compared with oil atomic emission spectrometer,the DOFP has high detection efficiency on the large ferromagnetic particles,and can well reflect the real particles concentration in oils.