The present invention provides optically pure (S)-(+)-N-methyl-3- (1-naphthaleneoxy)-3- (2-thienyl) propanamine, a compound of formula 1, and optically pure (S)-isomer of compound of formula 4, wherein R₁ and R₂ both are methyl or R₁ is methyl and R₂ is benzyl or substituted benzyl group and process for preparation thereof. In another aspect the present invention provides a process for preparation of an acid addition salt of compound of formula 1.