In the light of φ-mapping method and topological current theory,the contribution of disclination linesto free energy density of liquid crystals is studied in the single-elastic constant approximation.It is pointed out thatthe total free energy density can be divided into two parts.One is the usual distorted energy density of director fieldaround the disclination lines.The other is the free energy density of disclination lines themselves,which is shown tobe centralized at the disclination lines and to be topologically quantized in the unit of kπ/2.The topological quantumnumbers are determined by the Hopf indices and Brouwer degrees of the director field at the disclination lines,i.e.thedisclination strengths.From the Lagrange’s method of multipliers,the equilibrium equation and the molecular field ofliquid crystals are also obtained.The physical meaning of the Lagrangian multiplier is just the distorted energy density. In the light of φ-mapping method and topological current theory, the contribution of disclination linesto free energy density of liquid crystals is studied in the single-elastic constant approximation. It is pointed out that that total free energy density can be divided into two parts. One is the usual distorted energy density of director fieldaround the disclination lines. The other is the free energy density of disclination lines themselves, which is shown to be centralized at the disclination lines and to be topologically quantized in the unit of kπ / 2. The topological quantumnumbers are determined by the Hopf indices and Brouwer degrees of the director field at the disclination lines, iethedisclination strengths.From the Lagrange’s method of multipliers, the equilibrium equation and the molecular field of liquid crystals are also obtained. The physical meaning of the Lagrangian multiplier is just the distorted energy density.