Due to absence the gravity, an ideal spherical liquid drop can be formed in the space environment that is useful for some experiments like space liquid science and onorbit material composing. But the coupled vibration with its supporting structure may cause an undesirable distortion of the drop, so understanding its oscillation characteristic is essential. Using RayleighRitz method, this paper investigated the free oscillation of a liquid drop in zero gravity environment, and presented the natural frequencies and modes of the drop oscillation. Although some researchers have studied the same problem using dynamical condition on a drop surface, the energy method can provide a way to further study the liquidstructure coupled vibration in the zero gravity space environment because the energy method is usually adopted to deal with the vibration of more complicated coupled systems.