The　induction　of　structural　modifications　by　focused　laser　pulses　in　bulk　transparent　materials　is　a　versatile　technique　for　the　microfabrication　of　photonic　devices.　In　this　paper,　the　influence　of　repetition　rate,　pulse　energy　and　number　of　pulses　was　studied　by　comparing　picosecond　laser　induced　structural　modifications　in　sapphire.　The　observed　structural　modifications　consisted　of　a　uniaxial　void　zone　surrounded　by　smooth　refractive　index　changes,　and　micro　cracks　followed　by　a　submicrometer　filamentary　track.　The　number　of　laser　pulses　was　found　to　be　the　primary　factor　in　causing　cracks　surrounding　the　focal　volume　while　pulse　repetition　rate　and　pulse　energy　were　observed　to　have　minor　roles　in　these　cracks.　However,　with　increasing　repetition　rate,　length　of　the　filamentary　track　decreased,　whereas　no　significant　difference　was　observed　in　the　length　by　changing　number　of　pulses.　Furthermore,　filamentary　track　length　has　increased　with　the　increase　of　laser　energy.　At　107　mu　J　energy,　a　maximum　filamentary　track　of　length　1180　mu　m　was　observed.