Disclosed is a process for producing a 4-nitrosodiphenylamine of the formula (2) ##STR1## wherein R.sub.1 and R.sub.2 independently represent hydrogen atom, methyl group, ethyl group, cyclohexyl group, methoxy group, ethoxy group or chlorine or bromine atom, or a salt thereof, comprising treating a diphenylamine represented by the formula (1) ##STR2## wherein R.sub.1 and R.sub.2 are as defined above with (i) a mixture of nitrogen oxides, (ii) a hydrogen halide and (iii) an aliphatic alcohol, wherein the atomic ratio of oxygen to nitrogen of the mixture of nitrogen oxides is more than 1.0 and less than 2.0. This process produces 4-nitrosodiphenylamine of the formula (2) or a salt thereof conveniently and effectively, and is industrially advantageous.