Transformer　oil　has　been　used　to　electrically　insulate　a　variety　of　electrical　equipment　including　circuit　breakers,　cables　and　power　transformers.　Its　insulation　properties　and　breakdown　characteristics　under　impulse　overvoltage　is　the　key　factor　to　determine　the　safety　of　system.　The　insulation　configurations　of　power　transformers　and　reactors　are　affected　by　the　electrode　surface　area　and　stressed　oil　volume.　Therefore,　it＇s　of　great　importance　to　establish　proper　mathematical　models　to　cope　with　the　scale　effects　in　transformer　oil　under　impulse　voltage.　In　order　to　study　the　scale　effects　in　transformer　oil,　an　impulse　voltage　generator　with　adjustable　pulse　width　was　set　up　to　generate　a　steep　front　voltage　with　a　relative　long　duration　impulse　voltage　(0.12/50　mu　s).　The　electrical　breakdown　characteristics　of　transformer　oil　under　variable　electrode　area　and　interelectrode　distance　have　been　studied　using　Weibull　distribution.　It　turns　out　that　Weibull　distribution　function　of　three-parameter　fits　better　than　two-parameter.　The　empirical　formulas　of　the　dependence　of　the　average　electric　strength　on　different　stressed　electrode　areas　and　interelectrode　distances　are　expressed　as　E=CA(eff)(-beta)(d=0.5mm)　and　E=Bd(-xi)(D=30mm)　with　C=1.819,　beta=0.286,　B=0.823,　xi=0.196　according　to　experimental　results.　By　comparison　between　the　calculated　A(eff)　and　the　pit　area　on　the　electrode　surface　after　discharge,　it　is　shown　that　the　Aeff　obtained　from　using　3-parameter　Weibull　distribution＇s　shape　parameter　m　is　tally　with　the　actual　pit　area.