The dispersion and leakage loss in Bragg fibers are investigated by using a 1D full-vectorial finite-difference method. The physical mechanisms of leakage loss are explained from the view point of the band gap diagram,and two factors which can lead to the increasing leakage loss are identified. One is the transition of dispersion curve from the stop band to the pass band,which results in the absence of the band gap confinement. The other is the metallic-waveguide-like cutoff which dominates the rapid increasing loss for HE₁₁ mode in the long wavelength.