Elucidating the protein folding mechanism is crucial to understand how proteins acquire their unique structures to realize various biological functions. With this aim, the folding/unfolding of small globular proteins has been extensively studied. Interestingly, recent studies have revealed that even such small prote(i)ns represent considerably complex processes. In this study, we examined the folding/unfolding process of a small a-helical protein, the B domain of protein A (BdpA), at equilibrium using two-dimensional fluorescence lifetime correlation spectroscopy with 10 mu s time resolution. The results showed that although the BdpA is a two-state folder, both the native and unfolded states are highly heterogeneous and the conformational conversion within each ensemble occurs within 10 mu s. Furthermore, it was shown that the average structures of both ensembles gradually change and become more elongated as the denaturant concentration increases. The analysis on two mutants suggested that fraying of the N-terminal helix is the origin of the inhomogeneity of the native state. Because the direct observation of the ensemble nature of the native state at the single-molecule level has not been reported, the data obtained in this study give new insights into complex conformational properties of small proteins.