We calculate nuclear parton distribution functions (PDFs), using the constituent quark model. We find the bounded valon distributions in a nuclear to be related to free valon distributions in a nucleon. By using improved bounded valon distributions for a nuclear with atomic number A and the partonic structure functions inside the valon, we can calculate the nuclear structure function in x space. The results for nuclear structure-function ratio
at some values of A, are in good agreement with the experimental data.