Many drugs and bioactive molecules contain N-methyl group. One of the most important ways to introduce N-methyl group into the molecule is Eschweiler–Clarke reaction of amine with formaldehyde and formic acid, which is used as a reductant. We found that it can be conducted using only formaldehyde, which is also a good reductant. The developed procedure can be applied to various secondary amines, the process is convenient, cheap and eco-friendly.