Reported herein is the development of Cp*Ir(III)catalyzed direct C-H arylation of arenes and alkenes using aryldiazonium tetrafluoroborates, the use of which as an aryl precursor and also as an oxidant via C-N-2 bond cleavage was a key to success in achieving a mild and external oxidant-free procedure. Mechanistic experiments and DFT calculations revealed the turnover-limiting step to be closely related to the formation of an Ir(V)-aryl intermediate rather than the presupposed C-H cleavage. Under the developed mild arylation conditions, a wide range of benzamides were smoothly arylated. In addition, synthetic utility of the current C-H arylation procedure was also demonstrated successfully for the (Z)-selective arylation of enamides and C8-selective reaction of quinoline N-oxides.