A method of predicting NOx generation amount may include calculating NO generation rate by using a combustion pressure of an engine and driving variables of the engine, obtaining NO generation period by using the combustion pressure of the engine, calculating NO generation amount based on the NO generation rate and the NO generation period, and predicting the NOx generation amount by obtaining NO2 generation amount based on a ratio between NO and NO2 according to the NO generation amount and a driving condition of the engine.