In an internal combustion engine, inside of an engine exhaust passage, a hydrocarbon feed valve (15) and an exhaust treatment catalyst (13) are arranged. On a substrate (45) of the exhaust treatment catalyst (13), a coat layer comprised of at least two layers of a top coat layer (46) and a bottom coat layer (47) is formed. The top coat layer (46) is comprised of an exhaust purification catalyst for reacting NOx contained in exhaust gas and reformed hydrocarbons, while the bottom coat layer (47) is comprised of an NOx absorption catalyst. The concentration of hydrocarbons which flows into the exhaust treatment catalyst (13) is made to vibrate within a predetermined range of amplitude and within a predetermined range of period. Due to this, NOx contained in exhaust gas and NOx desorbed from the NOx absorption catalyst (47) are reduced in the exhaust purification catalyst (46).