Provided is a method of preparing 4,4′-dinitrodiphenylamine (4,4′-DNDPA) in high yield by reacting 4-nitroaniline with excess nitrobenzene via the NASH reaction, and a method of preparing 4,4′-bis(alkylamino)diphenylamine (4,4′BAADA) in high yield and purity by hydrogenating the resulting 4,4′-DNDPA with a ketone compound in the presence of hydrogen and hydrogenation catalyst. The disclosed process is simple, allows selective preparation of 4,4′-DNDPA without byproducts, and thus allows preparation of 4,4′-BAADA in high yield without a complicated purification procedure.