Abstract (1) A new thermodynamic relationship applicable to dilute solution of rigid rod-like molecules was derived, employing a lattice model. (2) Small deviation from Zimm’s result for rigid rod was discussed here, considering the effect of size and shape of solvent and solute. (3) Although quantitative discussion is impossible because of the difficulty of the estimation of the size of the submolecule, it may be said that the contribution of entropy of dilution to A2 (the second virial coefficient in osmotic pressure expression as power series in concentration) will be less for rigid rod-like molecules than for coiled molecules, and furthermore, the difference between such systems as cellulose derivatives in a series of solvents will be very small.