Power　electronic　equipments　are　taking　more　and　more　important　part　in　many　fields　such　as　power　supply,　power　system　and　renewable　energy　source　and　etc.　Acted　as　power　supply　or　motor　driver,　the　power　electronic　equipment　has　a　great　impact　on　the　systems＇　reliability.　So　the　studies　on　the　fault　diagnosis　for　power　electronics　circuit　have　significant　theory　value　and　practical　value.　Based　on　random　forests,　a　new　on-line　fault　diagnosis　method　for　power　electronic　circuit　is　put　forward　in　the　paper.　Although　there　are　several　existed　techniques　for　faults　diagnosis　of　power　electronic　circuit,　the　research　on　random　forests　algorithm　(RFA)　is　meaningful　and　necessary　because　of　its　fast　execution　speed,　the　characteristic　of　tree　classifier,　and　high　process-speed.　A　three-phase　SCR　rectifier　circuit　with　inductive　load　is　used　as　an　example　to　illustrate　the　new　approach　of　fault　diagnosis.　The　result　of　the　application　indicates　that　the　method　can　diagnose　faults　of　three-phase　rectifier　effectively.This　diagnosis　method　also　can　be　promoted　to　other　form　circuit,　and　possesses　vast　application　foreground.　©　2009　IEEE.