The hydrodynamic radius（Rh）of bovine serum albumin（BSA）was measured with the Zetasizer Nano instrument.The influences of pH，ionic strength and surfactants were investigated，and the real-time measurement of BSA size was used to monitor the thermal denaturation and urea denaturation of BSA.The results indicated that Rh showed a typical “U” form with the increase of pH.In acid solution，with increasing ionic strength the Rh decreased a little first，and then increased remarkably.At neutral pH the influence of ionic strength could be neglected.The adsorption of ionic surfactants on BSA molecular surface resulted in the change of Rh.For the thermal denaturation of BSA，the rate of denaturation increased with the increase of ionic strength.When sodium dodecyl sulfate（SDS）was added，the melting point Tmof BSA could increase.For urea denaturation，urea could cause BSA molecule to expand but also restrain the extension of peptide chains.By adding dithiothreitol（DTT），the Rh of BSA increased gradually with denaturation time.The results indicated that this convenient measurement of Rh could be used for characterizing the molecular size of protein，and monitoring the change of protein molecule during the denaturation process.