  We consider the modified Korteveg de Vriez equation on the whole line.
Initial data is real and step-like, i.e. $q(x,0)=0$ for $x\geq0$ and $q(x,0)=c$
for $x<0$, where c is arbitrary real number. The goal of this paper is to study
the asymptotic behavior of the initial-value problem's solution by means of the
asymptotic behavior of the some Riemann\textendash Hilbert problem. In this
paper we show that the solution of this problem has different asymptotic
behavior in different regions. In the region $x<-6c^2t$ and $x>4c^2t$ the
solution is tend to $c$ and 0 correspondingly. In the region $-6c^2t<x<4c^2t$
the solution takes the form of a modulated elliptic wave.
