Diazo compounds had been widely employed as a class of environment friendly and highly efficient coupling reagents in transition-metal-catalyzed C-H bonds insertion reactions. Recently, C-H alkylation of various arenes and heteroarenes has been achieved enormous successes using diazo compounds as the alkylation reagents under a new catalytic model. The recent progress in diazo compounds mediated transition-metal-catalyzed C-H alkylation and tandem C-H alkylation/cyclization is reviewed, including reaction mechanism and synthetic applications. Furthermore, the prospects of this reaction are also discussed.