The direct synthesis of isocoumarin skeletons has been realized through the Rh(III)-catalyzed [3 + 3] annulation of sulfoxonium ylides with iodonium carbenes. The synthetic protocol was constructed efficiently with broad functional group tolerance and mild reaction conditions. This reaction can be formally viewed as the result of C-H activation, carbene insertion and nucleophilic addition processes. Furthermore, the further conversions of the product and gram-scale reactions were also demonstrate to support the effectiveness of the synthesis protocol.