This paper describes an optical fibre magnetic field sensor system, developed by us based on previous work, for measuring D. C. or low frequency fields. In the measurement of low frequency magnetic fields based on the principle of magneto striction, such as the magnetic field of the earth, one of the main problems is the noise induces by the temperature fluctuations and low frequency vibrations. These low frequency noises can be reduced by letting the signal field to be carried by a high frequency carrier, provided by an oscillator. This was done in the work of P. Zhanzucchi[ 1] . In this system, two magneto strictive rods were used as sensor head in order to maintain some balance of the two arms. Based on this and other work [ 1] ,[ 2] ,[ 3] , we develop a sensor system which uses only one magneto strictive rod, instead of two, as sensor head. Our system has the main advantages of the system of Zhanzucchi, but is much simple in its structure.