A method for predicting a NOx amount may include detecting an O2 amount in an intake air, calculating a reference O2 amount in the intake air according to a driving condition of an engine, calculating a reference NOx amount contained in an exhaust gas according to the driving condition of the engine, and primarily correcting the reference NOx amount based on the detected O2 amount in the intake air and the reference O2 amount in the intake air according to the driving condition of the engine.