The denitration of nitric acid with formaldehyde is an efficient method for decreasing and eliminating the nitric acid content in waste water along with recovery of nitrogen oxides leaving no added reagent behind. Due to rapid reaction rate the stoichiometric ratios of the reaction rather than reaction rates are needed for practical application. The reaction of formaldehyde with nitric acid has been studied at temp. 100 ℃ in a semi-batch reactor and obtained an equation, α=1.02+0.207 C_HNO3 to determine the denitration ratios which is effective over the nitric acid conc. of 1.5-13 M. The relative errors between calculated values of ratios and the experimental data obtained for continuous reaction at temps 100 ℃ and 98 ℃ have been found less than 4 % and 7 % respectively. Also the absorption of NO₂ gas by nitric acid prior to denitration has contributed to a substantial reduction of induction period of the reaction