Removal of nitrogen oxides (NOx) from flue gases is crucial for solving the problems of acid rain and air pollution. Selective catalytic reduction of nitrogen oxides with ammonia as a reducing agent is one of the best prospective processes for cleaning the stack gas from stationary sources. Manganese oxides show high activity for NOx reduction with NH3 at low temperature. The kinetics of NOx reduction has been investigated with natural manganese ore and manganese oxides. The reaction order of manganese oxides for NO is 0.5. It can be concluded that the reduction mechanism of NOx with manganese oxides depends on Eley-Rideal and Langmuir-Hinshelwood mechanism simultaneously.