The present invention relates to cells or plants that can produce polylactate or its copolymers and to a method for preparing polylactate or its copolymers using the same. More specifically, cells or plants with the ability to produce polylactate or hydroxyalkanoate-lactate copolymers comprise both a gene encoding an enzyme that converts lactate into lactyl-CoA and a gene encoding polyhydroxyalkanoate (PHA) synthase which uses lactyl-CoA as a substrate. Also described is a method for preparing polylactate or hydroxyalkanoate-lactate copolymers which comprises culturing the cells in a medium containing lactate or lactate and various hydroxyalkanoates or culturing the plants. Effective preparation of hydroxyalkanoate-lactate copolymer which comprises various hydroxyalkanoates as well as polylactate, using the cells or the plants, is disclosed.