A new concept called the bending strength K of molecule is introduced, which can be calculated from the concret structure of molecular chemical bonds and the force constants of th: bonds, and the method of calculation is given. For example for PAA, anisaldehyde azine and BA, K is ３．１７×１０4，２．９３×１０4，４．１７×１０4ｎｍｋBＫ， respectively, from calculations. The elastic repulsive potential between two molecules is deduced and the statistical method for the system of elastic rod-like molecules is presented by means of cell model. A detailed calculation is made for PAA. The microscopic mechanism for forming nematic order is discussed and it is concluded that the attractive potential is aomininant, but the effect of steric repulsive potential is also important.