  Let $(H(n))_{n \geq 0} $ be a $2-$dimensional Halton's sequence. Let $D_{2} (
(H(n))_{n=0}^{N-1}) $ be the $L_2$-discrepancy of $ (H_n)_{n=0}^{N-1} $. It is
known that $\limsup_{N \to \infty } (\log N)^{-1} D_{2} ( H(n) )_{n=0}^{N-1}
>0$. In this paper, we prove that $$D_{2} (( H(n) )_{n=0}^{N-1}) =O( \log N)
\quad {\rm for} \; \; N \to \infty ,$$ i.e., we found the smallest possible
order of magnitude of $L_2$-discrepancy of a 2-dimensional Halton's sequence.
The main tool is the theorem on linear forms in the $p$-adic logarithm.
