Disclosed is a process for commercial preparation of 3-(methylamino)-1-(2-thienyl)propan-1-ol (hereinafter abbreviated as "MMAA") of the formula below: ##STR00001## The process is carried out by the diastereomeric salt formation method using optically active mandelic acid or its derivatives, or an optically active tartaric acid derivative as the resolving agent. The product compound, diastereomeric satls, is useful as the intermediate for producing pharmaceuticals, such as duloxetine.