Dimethylamine is a common impurity in methylamine, a reagent widely used in drug synthesis. The presence of dimethylamine is of particular concern because it can form N-nitrosodimethylamine, a known human carcinogen, through nitrosylation. For these safety reasons, regulatory agencies have emphasized the establishment of sensitive and accurate analytical method to control residual dimethylamine during the process. However, methylamine and dimethylamine have similar molecular structure, similar physicochemical properties, and low UV absorption, making selective analysis difficult under general HPLC conditions. Therefore, this study introduces a derivatization preparation method using 2,4-dinitrofluorobenzene and optimized HPLC analytical conditions to selectively detect dimethylamine in methylamine. The developed method enables reliable and accurate detection, providing a valuable tool for quality control and regulatory compliance in drug synthesis processes.