The effects of Ca doping on the oxygen ion diffusion and phase transition in oxide ion conductors La2Mo2O9 has been investigated by low frequency internal friction measurement and X-ray diffraction. It is found that the two relaxation subpeaks at lower temperature, which are associated with the oxygen ion diffusion, and the phase transition peak at higher temperature were affected seriously by Ca doping. With the increase of Ca doping content, the peak height of the relaxation peak decreases, but the activation energy increases at first and then decreases. From the influence of Ca doping on the peak height and activation energy of the relaxation peaks, it is deduced that Ca doping can make the oxygen ion diffusion easier in some doping content. As for the phase transition around 833 K, substituting La3+ cation in some degree with divalent Ca2+ can suppress it and maintain the high temperature phase to lower temperature, which help to improve the oxygen ions conducting property.