An indoor unit of an air conditioner has an improved discharge structure of air. The indoor unit of an air conditioner includes a panel which defines an external appearance thereof and has an opening, a mixed flow fan which is located within the panel, a heat exchanger unit which is located rearward of the mixed flow fan, an inlet port which is located rearward of the heat exchanger unit so that air is suctioned from rearward of the heat exchanger unit to be introduced into the mixed flow fan, an outlet port which is exposed forward of the panel through the opening so that the air passing though the mixed flow fan is discharged through a front portion of the panel, and a plurality of circular louvers to open and close the outlet port by moving outwards from a center of the outlet port.