An unexpected oxidative radical cyclization of 3(5)-aminoazoles and aromatic aldehydes with di-tert-butyl peroxide (DTBP) is described. This established protocol enables the assembly of privileged 7-methylpyrazolo[1,5-a]pyrimidines as well as 7-methyl-[1,2,4]triazolo[1,5-a]pyrimidines with excellent regioselectivity and functional group tolerance, where DTBP firstly emerges as the C2 cyclic unit rather than the usual methyl radical source. The reaction is further highlighted by the late-stage modifications of natural products and pharmaceuticals.