The reaction of N(2)H(4) with OH has been investigated by quantum chemical methods. The results show that hydrogen abstraction mechanism is more feasible than substitution mechanism thermodynamically. The calculated rate constants agree with the available experimental data. The calculated results show that the variational effect is small at lower temperature region, while it becomes significant at higher temperature region. On the other hand, the small-curvature tunneling effect may play an important role in the temperature range 220-3000 K. Moreover, the calculated rate constants show negative temperature dependence at the temperatures below 500 K, which is in accordance with Vaghjiani's report that slightly negative temperature dependence is found over the temperature range of 258-637 K. The mechanism of the major product (N(2)H(3)) with OH has also been investigated theoretically to understand the title reaction thoroughly.