The significant structure theory of liquids is satisfactorilly applied to liquid $He^4$. The Bose-Einstein partition function is used for the gas-like molecules. For the construction of the solid-like partition function the two-fluid model is applied. The Debye partition function is used for the solid-like molecules of normal fluid component, and the solid-like molecules of superfluid component are considered as the ground state molecules without having the positional degeneracies. The kinetic zero-point energy is introduced into the partition function, thereby extending the applicability of the significant sturectue theory of liquids to the extremely low temperature region of liquid $He^4$. The Bragg-Williams approximation of order-disorder phase transition is applied to the - transition. The partition function of liquid $He^4$ is obtained as the product of conventional partition function for the liquid and the configurational one for the -transition. The Debye partition function is directly calculated by the series expansion. The BoseEinstein partition function is calculated numerically, and the values of the Bose-Einstein intergrals are directly calculated by the approate series expansion. The empirical formula deduced from the experiments by Andronikashivili is used to calculate the fractions of both normal fluid and superfluid components. Thermodynamic properties of liquid $He^4$ are calculated from the proposed partition function. There are reasonable agreements between calculated results and available experimental data from literatures. The calculated surface tension using the iteration method yields good results and indicates that the main contribution to all of the surface tension below $3^\circ K$ is only top two or three layers of liquid. However, at higher temperature, the contribution may be extended to a little more layers.