The density of states in the gap of hydrogenated amorphous silicon prepared by DC glow discharge decomposition has been investigated through field effect measurement. The conductance change versus the field voltage reaches to $~10^6$. We obtain the mobility gap and the flat band voltage from the data. The mobility gap of the undoped a-Si:H film prepared at $310^\circ$C is 2.0 eV while the activation energy is 0.73 eV. The density of states in the gap of the sample at the Fermi level is about $2\times10^{17}eV^{-1}cm^{-3}$, and increase monotonously approaching to the mobility edge.
