A direct ab initio dynamics method has been employed to investigate the hydrogen abstraction reaction N2H4 + H --> N2H3 + H-2, which is predicted to have four possible reaction channels caused by different structures of N2H4 and different positions of hydrogen atom attack. The structures and frequencies at the stationary points and along the minimum energy paths (MEPs) are determined using the UMP2/6-31 + G(d,p) method. Energetic information is further refined at the PMP4/6-311 + G(3df,2pd)//UMP2/6-31 + G(d,p) level of theory. The rate constants are calculated using the improved canonical variational transition state theory with the small-curvature tunneling correction (ICVT/SCT) in the temperature range of 220-3000 K. The calculated results show that in the lower-temperature range, the most favorable reaction channels are reaction I [N2H4(a) + H --> TS(al) --> N2H3(a) + H-2] and reaction 3 [N2H4(b) + H --> TS(b1) --> N2H3(b) + H-2], while in the higher-temperature range, reaction 2 [N2H4(a) + H --> TS(a2) - N2H3(a) + H-2] and reaction 4 [N2H4(b) + H --> TS(b2) - N2H3(b) + H-2] become more important. The calculated total rate constants of the four reaction channels are in good agreement with the available experimental data.