We investigate the relative diffraction efficiency of a volume hologram where a spherical wave is used as the reference beam. Through theoretical analysis we find that the cross-talk performance of a volume hologram with a spherical reference wave is somewhat better than that with a planar reference wave. This phenomenon is explained and demonstrated. After comparing the readout images by a spherical reference wave with those by a planar reference wave, we also verify that a spherical reference wave is of little effects on the readout image quality. These features imply not so rigorous requirements on the parallelism of the reference beam in a volume holographic memory. In addition, a phenomenal analytical expression to approximately calculate the relative diffraction efficiency of a volume hologram with a spherical reference beam is presented.