An NOx purification system and a method for control of the NOx purification system. Two oxidation catalysts are provided upstream of an ammonia solution supply unit, and an in-cylinder fuel injection is so controlled that hydrocarbon in exhaust gas is mainly oxidized by an upstream one of the oxidation catalysts and nitric oxide in the exhaust gas is mainly oxidized by a relatively downstream one of the oxidation catalysts. NOx in the exhaust gas is purified by feeding an ammonia solution to a selective reduction NOx catalyst. The system adjusts to the concentrations of HC, NO and NO2 going to the selective reduction NOx catalyst to proper values so as to purify the NOx efficiently by the selective reduction NOx catalyst, and improve NOx purification performance in a lower temperature range.