An exhaust purification system of an internal combustion engine comprises an air-fuel ratio control device controlling an air-fuel ratio of the inflowing exhaust gas to a target air-fuel ratio. If a first judged condition wherein an estimated amount of change of an oxygen storage amount of the catalyst when the target air-fuel ratio is maintained at the first set air-fuel ratio is equal to or greater than a first reference amount, and a second judged condition wherein a difference of an air-fuel ratio detected by the upstream side air-fuel ratio sensor and a stoichiometric air-fuel ratio is equal to or greater than a difference of a second judged air-fuel ratio and a stoichiometric air-fuel ratio are satisfied, the air-fuel ratio control device switches the target air-fuel ratio from the first set air-fuel ratio to the second set air-fuel ratio when the first judged condition and second judged condition are satisfied.