The tert-butyl hydroperoxide (TBHP)-promoted sequential carboxamidation and aromatisation of isonitriles with formamides was developed. This procedure tolerates a series of functional groups, such as methyl, fluoro, chloro, acetyl, methoxy carbonyl and cyano. This reaction involved the addition of formamide radicals to isonitriles and sequential C-C bond formation by intramolecular aromatic cyclisation, leading to phenanthridine 6-carboxamides in moderate to good yields.