Proline isomerization catalyzed by peptidyl-prolyl cis-trans isomerase (PPI) in vivo is a limited procedure in protein folding. In order to study the catalyzing activity of PPI on the refolding of recombinant proteins in vitro,PPI is purified from pig kidney.and is investigated the effects of the enzyme on catalyzing the refolding process. Results indicate that PPI increases the folding rate without increasing the correct folding ratio and specific activity.and PPI has a high catalyzing activity even at very low concentration.