Abstract: Periodic disturbances in prime movers and governors（mechanical parts） are major causes for forced oscillations. Based on the Phillips-Heffron model of multi-machine systems, the transfer function matrix from disturbances in mechanical parts to mechanical power is derived, which is proven to be inherently diagonally dominant with equipment parameters in normal range. According to the property, the mechanical part of the generator whose mechanical power has the largest power spectral density is the possible oscillation source for oscillations caused by periodic disturbances, malfunctions and parameter abnormities in mechanical parts. Meanwhile, the influences of periodic disturbances in loads and exciters on mechanical power are considered. The transfer function matrices from loads and exciters to mechanical power are also derived, showing that it is reasonable to identify the generator with the largest power spectral density of mechanical power as oscillation source from the theory aspect. Demonstrative examples of 10-machine 39-bus system and North China Power Grid show the effectiveness and simplicity of the proposed location method.