Standard bulk methods of probing folding energetics does not measure the energetic change of the folded and unfoled states during protein denaturation directly.The way of baseline extrapolation to approximate the energetics of end states introduces significant uncertainty of protein folding energetics because many experiments indicate that the end state energetics changes in complex ways during denaturation.Alternatively single molecule methods,i.e.FRET,provide the capability to distinguish mixed states of protein molecules during denaturation processing by mesauring individual molecules.Thus we employed the SM-FRET to study the end state energetics of a 2-state model protein SH3.Especially we estimate the electrostatic contribution to the energetics of end states by using the ionic (GdmC1) and non-ionic (urea) chemical denaurants that perturb the protein energetics with different effect.