The bandwidth characteristics of fiber Bragg grating (FBG) made with femtosecond laser radiation are studied theoretically and experimentally. The simulation results show that the FBG bandwidth increases with refractive index modulation increasing, while it decreases dramatically and then saturates with grating length increasing. FBGs are fabricated in silica fiber with femtosecond laser radiation and a phase mask and then characterized by spectral analyser. FBG bandwidth remains constant when the laser power is low. However, when the laser power is large enough, FBG bandwidth increases with laser power increasing. With the exposure time increasing, FBG bandwidth increases initially and then become saturated. These results can be attributed to the change of the refractive index modulation and the grating length with femtosecond radiation.