An integro-differential equation is evolved for the radial distribution function in liquid He4 at absolute zero assuming a particular form of the ground state wave function, the validity of the super-position approximation and a simple relation between the radial distribution function and the wave function. For a system of hard-spheres this equation is transformed into an integral equation for the Fourier transform of the radial distribution function which is closely related to the form factor, and an approximate solution is obtained under the linear approximation. The calculated form factor is compared with the experimental data.