Flow stress decreases by superimposition of ultrasonic oscillatory stress during plastic deformation. The amplitude dependent internal friction is investigated together with the stress decrement due to the superimposition of oscillation during the plastic deformation of KCl single crystals doped with KBr, KI or SrCl2. The relation between the amplitude dependent internal friction and the stress decrement is divided into three regions for KCI doped with KBr or KI. The internal friction is proportional to the stress decrement in the first region up to the first bending point at the lower stress amplitude. The stress decrement at the first bending point is proportional to the square root of KBr or KI concentration. The internal friction of KCl-SrCl2 has the amplitude independent part. The stress decrement at the onset of increasing of internal friction is independent of SrCl2 concentration. The amplitude dependent internal friction of KCl-KBr and KCl-KI consists of the part due to the impurity and that due to forest dislocation. The amplitude dependent internal friction of KCl-SrCl2 is only due to the forest dislocation.