A clutch unit of an automatic transmission having first and second rotation elements may include a spline gear connected to the first rotation element, a clutch drum having an internal circumference spline-engaged with the spline gear to be movable in an axial direction, a clutch hub fixedly connected to a second rotation element, a plurality of clutch plates spline-engaged with the clutch drum, a plurality of clutch disks alternatingly disposed between the plurality of clutch plates and spline-engaged with the clutch hub, a supporting portion fixed to a transmission housing of the automatic transmission and configured to support the plurality of clutch plates, and a piston disposed in an interior of the transmission housing to form a hydraulic pressure chamber with the transmission housing, configured to have a rolling contact with the clutch drum interposing a bearing, and configured to transmit a pressurizing force to the plurality of clutch plates and the clutch disks through the clutch drum.