  We analyze experimental data of nuclear structure function ratios
$F_2^A/F_2^D$ for obtaining optimum parton distribution functions (PDFs) in
nuclei. Then, uncertainties of the nuclear PDFs are estimated by the Hessian
method. Parametrization of nuclear parton distribution is investigated in the
leading order of $\alpha_s$. The parton distribution are provided at $Q^2=1
GeV^2$ with a number of parameters, which are determined by a $\chi^2$ analysis
of the data on nuclear structure function. From the analysis, we propose parton
distributions at $Q^2=1 GeV^2$ for nuclei from deuteron to heavy ones with a
mass number $A\sim 208$.
