Provided is a mutant of propionyl-CoA transferase from Clostridium propionicum that can convert lactate into lactyl-CoA with high efficiency in a method of preparing a polylactate (PLA) or PLA copolymer using microorganisms. Unlike conventional propionyl-CoA transferase which is weakly expressed in E. coli, when a mutant of propiony-CoA transferase from Clostridium propionicum is introduced into recombinant E. coli, lactyl-CoA can be supplied very smoothly, thereby enabling highly efficient preparation of polylactate (PLA) and PLA copolymer.