Method for predicting nox amount amd exhaust system using the same

The present invention relates to an exhaust device to control the amount of reducing agent to be injected in accordance with the amount of nitrogen oxides to nitrogen and the method to accurately predict the amount of oxides contained in the exhaust gas generated by the engine or controlling the combustion atmosphere. A method of predicting the amount of nitrogen oxide in accordance with an embodiment of the present invention includes the steps of detecting the oxygen (O2) in the intake air amount; Calculating a reference O2 amount of intake air according to the operating condition of the engine; Calculating the amount of the reference nitrogen oxide (NOx) contained in exhaust gas in accordance with the operating condition of the engine; And reference of the intake air according to the operating conditions of the engine and the amount of O2 of the detected intake air comprising: first correction drive the criteria NOx amount according to the amount of O2; may include.