  Recently there has been disagreement between various experiments about the
value of the proton radius which is known as the proton radius puzzle. Since
the proton is not a point particle the charge radius of the proton depends on
the charge distribution (the form factor) of the partons inside the proton.
Since this form factor is a non-perturbative quantity in QCD it cannot be
calculated by using the perturbative QCD (pQCD) method but it can be calculated
by using the lattice QCD method. In this paper we formulate the lattice QCD
method to study the charge radius of the proton. We derive the non-perturbative
formula of the charge radius of the proton from the first principle in QCD
which can be calculated by using the lattice QCD method.
