In this paper, modulation sidebands about the tooth meshing frequency in a two stages planetary gear reducer vibration are investigated. The two stages gear reducer is developed using double-enveloping cycloid drive with a K-H-V planetary gear. Firstly, the double-enveloping cycloid drive and the meshing characteristics are introduced. Secondly, the meshing frequencies and the sidebands of this prototype gear reducer are calculated based on a model, which explains the modulation in terms of the motion of the planets relative to the location of the transducer. Thirdly, the experiment vibration spectrums are compared with the calculated results. And the comparisons between the simulation and calculation results show good agreement.