Optofluidics has recently become a new active research area, which combines the discipline of optics with that of microfluidics and is an exciting platform for exploiting the optical properties of fluids in optics and photonics. On the other hand, magnetic fluid (or ferrofluid) is a kind of attractive fluid materials that possesses several unique magneto-optical properties, such as tunable refractive index, magneto-volume variation and so on. Due to the fluidity of magnetic fluids/ferrofluids, they are easy to be integrated with fibers or infiltrated into microholes/microcavities. Microfiber possesses the characteristic of low dimension and large evanescent field assigned to the unique geometry. Photonic crystal microcavity is a good candidate for sensing because of its high quality factor. Therefore, combining magnetic fluids/ferrofluids with microfiber or photonic crystal structures is promising for realizing novel optofluidic magnetic field sensor. In this talk, we will present the magnetic field sensing with optofluidic techniques. The magnetic fluids/ferrofluids are used as the magnetic field sensitive fluids. The sensing structures include microfiber, microfiber coupler, microfiber Sagnac loop, microfiber knot resonator and photonic crystal microcavity.