The present invention provides a PHA (polyhydroxyalkanoate) synthase useful in a process for preparing a PHA, a gene encoding the enzyme, a recombinant vector comprising the gene, a transformant transformed by the vector, a process for producing a PHA synthase utilizing the transformant and a process for preparing a PHA utilizing the transformant. A transformant obtained by introducing a PHA synthase gene from Pseudomonas putida P91 strain into a host microorganism is cultured to produce a PHA synthase or PHA.