Action spectroscopy (AS) of single molecules using a scanning tunneling microscope (STM) is presented. The AS obtained from the motion and reaction of a molecule, is applicable to understand molecular dynamics at metal surfaces. Combined study with the generic theory of STM-AS, not only the energy for the vibrational modes, but also versatile numbers such as reaction order, reaction constant, time constants of mode coupling between the excited vibrational mode and the reaction-coordinate mode, etc., can be obtained.