  In this paper, we study the Newtonian limit for relativistic Boltzmann
equation in a periodic box $\mathbb{T}^3$. We first establish the
global-in-time mild solutions of relativistic Boltzmann equation with
uniform-in-$\mathfrak{c}$ estimates and time decay rate. Then we rigorously
justify the global-in-time Newtonian limits from the relativistic Boltzmann
solutions to the solution of Newtonian Boltzmann equation in
$L^1_pL^{\infty}_x$. Moreover, if the initial data of Newtonian Boltzmann
equation belong to $W^{1,\infty}(\mathbb{T}^3\times\mathbb{R}^3)$, based on a
decomposition and $L^2-L^\infty$ argument, the global-in-time Newtonian limit
is proved in $L^{\infty}_{x,p}$. The convergence rates of Newtonian limit are
obtained both in $L^1_pL^{\infty}_x$ and $L^{\infty}_{x,p}$.
