A new radical-induced dehydrogenative heterocyclization of arylamines with α-diazo carbonyls has been established under metal-free oxidative conditions, enabling two-fold C(sp²)–H bond amination to access a wide range of functionalized 1,2,4-triazine derivatives with generally good yields by combining KI/tert-butyl peroxide (TBHP). The present protocol features wide substrate scope, commercial accessibility, and mild reaction conditions. Mechanistic details of this radical process are rendered by conducting systematic theoretical calculations.