A co doped Bi substituted rare earth iron garnet single crystal YbBi∶YIG was grown from high temperature solution, and was used first time for magneto optic optical fiber current sensors. The crystal has the specific Faraday rotation angle 404deg/cm, magneto optic figure of merit 25.8deg/dB, saturation magnetic field 1400Oe, and temperature coefficient 4.2×10 -4 K -1 (at room temperature, λ=1.55μm). It has larger Faraday rotation angle and magneto optic figure of merit and lower temperature sensibility than pure YIG.A laboratory prototype sensor was established using the YbBi∶YIG crystal as Faraday rotator element, and its sensing performance was tested under a 50Hz frequency AC. The tested results show that the sensor system has high accuracy, sensitivity and good linearity.