A fabricating method of a flexible circuit board includes the following steps. The metal carrier foil with metal oxide layer on its surfaces is provided first. The metal oxide layer is formed from the spontaneous oxidization of the metal carrier foil in ambient air and provides passive protection in a sulfuric acid solution or an acidic copper sulphate solution. A conductive seed layer is electroplated onto the metal oxide layer. A flexible insulating layer is formed onto the conductive seed layer by performing a polyimide casting process. The metal carrier foil is then peeled off from the conductive seed layer, which is supported by the insulating layer. A patterned circuit is formed on the insulating layer by performing photoresist coating, developing and etching.