The optical fiber magnetic field sensors have the advantages of high sensitivity, small size, corrosion resistance, and anti-electromagnetic interference, overcoming the drawbacks of the traditional magnetic field sensor, and play a crucial role in many industries, including military, business, and power grid. Two different types of long-period fiber gratings (LPFGs) have been fabricated on both traditional single-mode fiber and photosensitive fiber using a carbon dioxide laser. The magnetic field sensors can be created by submerging the LPFG into the magnetic fluid. The resonance wavelength of the LPFG will be shifted when being applied to an external magnetic field. The attained greatest sensitivity of the wavelength shift was 126.5 pm/mT when the magnetic strength varies in the range of 1.6-25.5 mT. The proposed magnetic fiber optic sensor has potential applications in magnetic or electric field systems.