An exhaust purification system of an internal combustion engine wherein: a hydrocarbon feed valve for feeding hydrocarbons is arranged in an engine exhaust passage (12a, 12b), an exhaust purification catalyst for reacting NO x contained in exhaust gas and reformed hydrocarbons is arranged in the engine exhaust passage downstream of the hydrocarbon feed valve, a downstream side air-fuel ratio sensor for detecting the air-fuel ratio of the exhaust gas is arranged in the engine exhaust passage downstream of the exhaust purification catalyst, the downstream side air-fuel ratio sensor is a type of sensor which has a solid electrolyte, electrodes respectively covering two side surfaces of the solid electrolyte, and a diffusion resistance layer covering one of electrodes and in which exhaust gas is guided onto the diffusion resistance layer, precious metal catalysts (51, 52) are carried on an exhaust gas flow surface of the exhaust purification catalyst and a basic exhaust gas flow surface part is formed around the precious metal catalysts (51, 52), characterized in that an upstream side air-fuel ratio sensor for detecting an air-fuel ratio of exhaust gas is arranged in the engine exhaust passage upstream of the hydrocarbon feed valve, the exhaust purification catalyst has a property of reducing the NO x which is contained in exhaust gas if a concentration of hydrocarbons which flow into the exhaust purification catalyst is made to vibrate by within a predetermined range of amplitude and within a predetermined range of period and has a property of being increased in storage amount of NO x which is contained in exhaust gas if the vibration period of the hydrocarbon concentration is made longer than the predetermined range, when hydrocarbons are fed from the hydrocarbon feed valve, an air-fuel ratio detected by the downstream side air-fuel ratio sensor changes to a rich side from a reference air-fuel ratio which is detected when hydrocarbons are not fed from the hydrocarbon feed valve, and an amount of hydrocarbons which are fed from the hydrocarbon feed valve and which slip through the exhaust purification catalyst is detected from an air-fuel ratio difference between an air-fuel ratio which is detected by the upstream side air-fuel ratio sensor and the reference air-fuel ratio which is detected by the downstream side air-fuel ratio sensor. a hydrocarbon feed valve for feeding hydrocarbons is arranged in an engine exhaust passage (12a, 12b), an exhaust purification catalyst for reacting NO x contained in exhaust gas and reformed hydrocarbons is arranged in the engine exhaust passage downstream of the hydrocarbon feed valve, a downstream side air-fuel ratio sensor for detecting the air-fuel ratio of the exhaust gas is arranged in the engine exhaust passage downstream of the exhaust purification catalyst, the downstream side air-fuel ratio sensor is a type of sensor which has a solid electrolyte, electrodes respectively covering two side surfaces of the solid electrolyte, and a diffusion resistance layer covering one of electrodes and in which exhaust gas is guided onto the diffusion resistance layer, precious metal catalysts (51, 52) are carried on an exhaust gas flow surface of the exhaust purification catalyst and a basic exhaust gas flow surface part is formed around the precious metal catalysts (51, 52), an upstream side air-fuel ratio sensor for detecting an air-fuel ratio of exhaust gas is arranged in the engine exhaust passage upstream of the hydrocarbon feed valve, the exhaust purification catalyst has a property of reducing the NO x which is contained in exhaust gas if a concentration of hydrocarbons which flow into the exhaust purification catalyst is made to vibrate by within a predetermined range of amplitude and within a predetermined range of period and has a property of being increased in storage amount of NO x which is contained in exhaust gas if the vibration period of the hydrocarbon concentration is made longer than the predetermined range, when hydrocarbons are fed from the hydrocarbon feed valve, an air-fuel ratio detected by the downstream side air-fuel ratio sensor changes to a rich side from a reference air-fuel ratio which is detected when hydrocarbons are not fed from the hydrocarbon feed valve, and an amount of hydrocarbons which are fed from the hydrocarbon feed valve and which slip through the exhaust purification catalyst is detected from an air-fuel ratio difference between an air-fuel ratio which is detected by the upstream side air-fuel ratio sensor and the reference air-fuel ratio which is detected by the downstream side air-fuel ratio sensor.