Abstract: Equilibrium denaturation of lysozyme by guanidinium chloride in the presence of betaine was investigated by tryptophan fluorescenceBetaine was used as a folding aid to enhance the renaturation of denatured-reduced lysozyme, the refolding kinetic behavior was studied at a low guanidinium chloride concentration by a competitive model of first-order folding reaction and third-order aggregationIt was found that betaine could shift in the transition midpoint in the guanidinium chloride induced equilibrium unfolding experiments, indicating that betaine could improve the thermodynamic stability of lysozymeIn the presence of betaine, the aggregation rate was decreased and the refolding rate was increased, the refolding yield could be increased.