Change of global protein structure as a function of concentration of a denaturant, guanidine hydrochloride (Gdn-HCl), was studied by using the laser-induced transient grating (TG) method for carboxymyoglobin (MbCO). From the time profile of the TG signal after the photodissociation reaction of the ligand, diffusion coefficients (D) of the protein were determined at various concentrations of the denaturant. The denaturation curve of MbCO monitored by D was compared with that monitored by the circular dichroism (CD) method. The -m value and DeltaG(N-U)(0) determined from the transition curve monitored by D are smaller than those obtained from the CD signal intensity. This noncoincidence of the two transition curves indicates that the global structure of Mb is still changing after the complete secondary structure (alpha-helices) deformation process. The smaller diffusion coefficient of unfolded MbCO compared to folded MbCO can be interpreted in terms of changes of the protein-water interaction and the surface roughness of the protein, which is due to the conformational change of the protein from the native to the unfolding state.