A method is provided to estimate NO2 concentration in exhaust gas flowing through an exhaust pipe of an internal combustion engine. The exhaust pipe has a first section and a second section, and is equipped with an aftertreatment device located between the first section and the second section, and the method includes, but is not limited to determining a parameter related to a NO2 concentration of exhaust gas flowing in the first section, determining an oxidization index expressive of a rate of NO contained in the exhaust gas which oxidizes into NO2 inside the aftertreatment device, and a reduction index expressive of a rate of NO2 contained in the exhaust gas which is reduced into NO inside the aftertreatment device, and calculating a parameter related to a NO2 concentration in the second section as a function of the parameter related to a NO2 concentration in the first section and of the oxidization index and of the reduction index of the aftertreatment device.