Photo-ablation of sapphire by ultrashort laser pulses was investigated by time-of-flight mass spectrometry. Experiments were performed on a laser fluence range below the single shot damage threshold. The dependence of emitted positive ion intensity on both the laser fluence and the number of laser pulses, hitting the same target site, was studied. In addition, the ion kinetic energy distribution was analysed. We find that the ablation is caused by surface explosion. The origin of this explosion, however, is still unknown.