Abstract: A process for stereoselective preparation of (E)-enolthioether derivatives comprising the steps of;(a) protecting free secondary hydroxy group of L-threonine with trialkylsilyl group; (b) the protected L-threonine derivatives (I) being degraded by ninhydrin to prepare (2R)-2-(trialkylsilyloxy)propanal (II); and (c) carting out the Honor-Wordsworth-Emons (HWE) reaction of the resultant compound (II) with stabilized phosphonate ylide, phosphonium ylide or phosphine oxide ylide which was obtained by treating the organic phosphine compound (III) with strong base to provide (E)-enolthioether derivatives (IV) is provided.The present invention has advantages in comparison with the prior art in that the costs required for the process can be lowered because L-threonine, an .alpha.-amino acid of low in price is used as a starting material and the process is simple; and the desired (E)-enolthioether derivatives can be prepared with high stereoselectivity.