The unfolding of alpha-momorcharin was systematically investigated using steady-state and time-resolved tryptophan fluorescence, circular dichroism and 8-anilino-1-naphthalenesulfonic acid (ANS) binding. These spectroscopic studies demonstrated that alpha-momorcharin unfolded through a compact folded intermediate state. The content of alpha-helix was increased, Trp192 approached closer to the side of active site and its rotational motion was restricted by being equilibrated with 2-3 M of guanidine hydrochloride. Furthermore, the binding of ANS with alpha-momorcharin was more suppressed to show that the hydrophobic parts would not be accessed to the protein surface but rather be sealed off in this specific conformation state. These results suggest that the structure of alpha-momorcharin holds the more compact conformation as an incipient state for unfolding, which is the sharp contrast to beta-momorcharin that gives the characteristics of the generally known molten globule state.