Abstract: This paper investigates the characteristics of surface mobility when power transmission over the contact area between sub-structures is considered. An analytical formula of the surface mobility of an infinite homogeneousplate over a rectangular contact area subject to a uniform, conphase force excitation is derived by using complex power method and the concept of structural intensity. This formula provides a theoretical tool for investigating power transmission over the contact area between sub-structures. The influences of the size, the dimension and the aspect ratio of the contact area on power transmission are analyzed and described according to the results calculated, that provides an effectively theoretical method for investigation of vibration isolation.