In　the　environment　of　electricity　market　the　reactive　power　supporting　by　power　plants　is　an　important　ancillary　service　and　is　also　an　effective　measure　to　ensure　the　secure　and　stable　operation　of　power　system,　improve　power　quality,　reduce　power　losses　and　make　the　electric　energy　transaction　smoothly　carried　out,　so　it　is　necessary　to　analyze　the　cost　of　reactive　power　supporting　service.　The　authors　synthetically　analyzed　the　compensating　cost　of　reactive　power　supporting　from　both　sides　of　power　plant　and　power　network.　According　to　the　operating　limit　of　generator　the　mechanism　of　producing　opportunity　cost　was　revealed　and　the　distribution　function　of　directive　cost　of　the　reactive　power　supporting　service　at　power　plant　side　was　obtained.　The　contribution　of　reactive　power　supporting　service　to　power　system　at　the　power　network　side　was　analyzed　and　the　system　contribution　cost　of　the　reactive　power　supporting　service　was　researched.　Under　the　environment　of　electricity　market　the　power　plants　should　gain　the　synthetic　economic　compensation　from　both　sides　for　the　reactive　power　supporting　service　being　provided　in　order　to　guide　the　investment　of　reactive　power　capacity　and　ensure　the　stable　and　secure　operation　of　power　system.