The femtosecond laser induced void array inside Al_2O_3 crystals was discussed. The void array was formed spontaneously under the irradiation of a single beam of infrared femtosecond laser which was focused at a fixed point inside the Al_2O_3 crystal sample. It was found that the regular voids only could be fabricated near the sample surface, which was different from the situation in CaF_2 single crystal reported before. The possible mechanism of the phenomena was also discussed.