The concept of directional binding energy(DBE) is proposed to describe the binding energy of the crystal and the ideal direction of the antiaction force against the surface tension, which derives the grain-boundary energy and also energy ratio (gamma(gb)/gamma(s)). It may provide a simple and lucid way to analyse the surface energy, grain-boundary energy and energy ratio from the correlative view point. It has been found that the grain-boundary energy can be derived only from the conceptual approach of DBE, irrespective of the dislocation model, which also makes it possible to determine the energy ratio with the misorientation angle. That is, the energy ratio (gamma(gb)/gamma(s)) is proportional to the misorientation angle, 2sin(k’theta/2) where k’ is a constant and theta is the misorientation angle.