A new atomic force microscopy (AFM)-based technique, single-molecule force spectroscopy (SMFS), was used to study the nanomechanics of Bombyx mori silk fibroin. Three types of force-extension curves were found in the system. A modified freely jointed chain (MFJC) model can fit two of the three types well, but the fit parameters are different. The third type of force curve, in which there exists a plateau, cannot be modeled by MFJC. These results may show that there are three kinds of conformations in the silk fibroin system and SMFS can "distinguish" different conformations of the polymer chains.