The objective of this study was to elucidate the effects of hydroxylpropyl-beta-cyclodextrin (HP-beta-CD) on the in vitro stability of insulin. It was found that HP-beta-CD had positive effects on the stability of insulin in acid and base and under high temperature conditions. Furthermore, use of HP-beta-CD could also increase the stability of disulfide bonds which are important to the conformation of insulin. Through H-1-NMR experiments it was found that the protective effect of HP-beta-CD was due to complexation with insulin. The results suggest that the presence of HP-beta-CD could improve the stability of insulin in different environments.