Abstract: The present invention relates to a novel and improved process for preparing 4-acyloxy-2-azetidinone derivatives having the following formula (I) which are useful as an intermediate for synthesis of penem or carbapenem compounds: ##STR1## in which R.sub.1 represents a hydroxy-protecting group; and R.sub.3 represents an acyl group, which comprises reacting an azetidinone derivative having the following formula (II): ##STR2## wherein R.sub.1 is defined as above and R.sub.2 represents an alkyl group or an aryl group, with a N-haloacylimide in the presence of an organic carboxylic acid or a salt of an organic carboxylic acid.The present invention uses the organic compound, N-haloacylimide, as the reactant rather than the heavy metal reactant used in the prior art, and thus provides an economical advantage and does not cause any problem related to safety and waste disposal after the reaction.