Streamer　and　breakdown　behavior　of　transformer　oil　under　impulse　voltage　are　investigated　with　experimental　observations　and　simulation.　The　optical　observation　by　using　Intensified　Charge-coupled　Device　(ICCD)　fast　snapshot　technique　indicates　that　streamer　channel　branching　is　of　a　stochastic　nature.　The　2-D　axially　symmetric　modeling　based　on　hydrodynamic　drift-diffusion　model　is　employed　to　depict　the　streamer　dynamics　of　transformer　oil.　The　experimental　and　simulation　results　reveal　that　bushy　negative　streamer　requires　higher　excitation　than　filamentary　positive　streamer.　Positive　streamer　travels　faster　than　negative　streamer　from　experimental　estimation.