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