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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.
%It is difficult to
%determine the antiquark distributions at medium $x$ and gluon
%distributions.
From the analysis, we propose parton distributions at $Q^2=1
GeV^2$ for nuclei from deuteron to heavy ones with a mass number
$A\sim 208$.
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