In order to solve the contradiction found in the Hall-Petch behavior in the ultra-finegrained- and nano-grained- materials, reinvestigation of the physical meaning of Hall-Petch constants has become necessary. The present work is hence attempted to theoretically construct the Hall-Petch slope constant, KH-P. This was carried out based on the principle of image analysis and previous internal energy approach. After analyzing several influencing factors on the KH-P, a model was established with 95% accuracy in predicting the KH-P value. In other words, this model, for the first time, has related the KH-P value to grain boundary surface energy,γ , shear modulus, G, and lattice constant, a0, via 1/ 2 0 K β G(2a γ ) H P = − .