We have developed an absorption spectrum based molecular ruler that measures the distance between two mercapto groups. The conformation of the molecular rulers changes with the distance, which induces an absorption spectrum change. DFT calculation has been carried out to elucidate the relation between the molecular conformation and the absorption spectrum. With this simple method, we can estimate the distance between two mercapto groups on the scale of 0.4-1 nm. This method can also be used to monitor conformation transition, which is demonstrated by a silver ion titration experiment.