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A magnetic sensor utilizing long-period fiber grating (LPFG) written by high frequency CO2 laser pulses in a D-shaped fiber and the magneto-optical effect of magnetic fluid is proposed. Because of the high evanescent field of the grating structure in D-shaped fiber, by immersing the LPFG with a period of 685 μm into a water-based magnetic fluid within a capillary tube, the redshift as high as 33.5 nm of the resonant wavelength of the grating is observed when the external magnetic-field intensity is 189.7 mT, resulting in a sensitivity of 176.4 pm/mT. The hysteresis effect is observed and explained by the characters of magnetic fluid. Such kind of sensors would find potential applications in magnetic sensing fields.