Azo compounds have attracted much attention in the past decades, due to their unique properties. Recently, the free radical strategy has been widely applied to the chemical transformations of azobenzenes, dialkyl azodicarboxylates, mainly including hydrogenation and functionalization of N=N double bond, and N-functionalization reactions via the cleavage of the N=N double bonds. In this review, we systematically summarized these reactions as well as their mechanisms are discussed.