The ultrashort pulse laser writing multi-layered data bits in fused silica for three-dimensional optical data storagethrough the mechanism of optical damage induced by multi-photon absorption is demonstrated. The 800nm,120fs Ti:Sapphire laser beam is focused into cubic samples with a 0.65 NA commercial microscope objective. Thesize of the recorded bits can be smaller than 0.5 × 0.5 × 2.5μm as the laser pulse energy is lower than 300nJand the corresponding bit data storage density is 0.2 × 1012 Tbits/cm3. Due to the aberration produced by themismatch of the refractive indices between the recording material and its immersion medium, the longitudinalsize of the recorded bit increases and the contrast of the bit to the surrounding uninfected zone decreases as thebit plane gets deeper. Hence, a relatively lower storage density of 0.05 × 1012 Tbits/cm3 is realized.