A method for predicting a NOx amount, comprising: determining an O2 amount in an intake air; determining a reference O2 amount in the intake air according to a driving condition of an engine; determining a reference NOx amount (ψ NOx,Exh,Ref) contained in an exhaust gas according to the driving condition of the engine; correcting the determined reference NOx amount (ψ NOx,Exh,Ref) based on the detected O2 amount (ψ O2,Intk) in the intake air and the determined reference O2 amount (ψ O2,Intk,Ref) in the intake air according to the driving condition of the engine; and secondarily correcting the corrected reference NOx amount according to an environmental factor, wherein the driving condition of the engine includes an engine speed, a currently engaged gear step, a current fuel injection amount, a temperature of the intake air supplied into a combustion chamber of the engine, and a coolant temperature, and wherein the secondarily correcting of the corrected reference NOx amount according to the environmental factor includes: determining a first correction coefficient according to a total air amount supplied into a combustion chamber of the engine, the engine speed, and the current fuel injection amount; determining a second correction coefficient according to the engine speed, the current fuel injection amount, and the coolant temperature; detecting a third correction coefficient according to the engine speed, the current fuel injection amount, and the temperature of the intake air supplied into the combustion chamber of the engine; and correcting the corrected reference NOx amount according to the first correction coefficient, the second correction coefficient, and the third correction coefficient. determining an O2 amount in an intake air; determining a reference O2 amount in the intake air according to a driving condition of an engine; determining a reference NOx amount (ψ NOx,Exh,Ref) contained in an exhaust gas according to the driving condition of the engine; correcting the determined reference NOx amount (ψ NOx,Exh,Ref) based on the detected O2 amount (ψ O2,Intk) in the intake air and the determined reference O2 amount (ψ O2,Intk,Ref) in the intake air according to the driving condition of the engine; and secondarily correcting the corrected reference NOx amount according to an environmental factor, wherein the driving condition of the engine includes an engine speed, a currently engaged gear step, a current fuel injection amount, a temperature of the intake air supplied into a combustion chamber of the engine, and a coolant temperature, and wherein the secondarily correcting of the corrected reference NOx amount according to the environmental factor includes: determining a first correction coefficient according to a total air amount supplied into a combustion chamber of the engine, the engine speed, and the current fuel injection amount; determining a second correction coefficient according to the engine speed, the current fuel injection amount, and the coolant temperature; detecting a third correction coefficient according to the engine speed, the current fuel injection amount, and the temperature of the intake air supplied into the combustion chamber of the engine; and correcting the corrected reference NOx amount according to the first correction coefficient, the second correction coefficient, and the third correction coefficient. determining a first correction coefficient according to a total air amount supplied into a combustion chamber of the engine, the engine speed, and the current fuel injection amount; determining a second correction coefficient according to the engine speed, the current fuel injection amount, and the coolant temperature; detecting a third correction coefficient according to the engine speed, the current fuel injection amount, and the temperature of the intake air supplied into the combustion chamber of the engine; and correcting the corrected reference NOx amount according to the first correction coefficient, the second correction coefficient, and the third correction coefficient.