Abstract: Input impendence characteristics of cosine loaded ultrasonic horn are studied.Input imped-ance of four types of loaded transformers is studied,and unified expressions of both input impedance and Möbius transformation parameters are given.At zero input reactance of ultrasonic horn,the resonance frequency equation is evaluated,and the magnification coefficient of loaded cosine longitudinal ultrasonic horn calculated.Based on the expression of input impedance,stability of the transformer and relative impedance equivalent are discussed.