\documentclass[12pt,a4paper]{report} 
\usepackage{amsmath,tikz}
\usepackage{subfigure}
\usepackage{graphicx}
\usepgflibrary{arrows}
\usepackage[colorlinks=true]{hyperref}
\usepackage{amssymb}
\usepackage{xepersian}
\settextfont[Scale=1.1]{XB Niloofar}
\setdigitfont{ParsiDigits}
\usepackage{amsmath,tikz} 

\tikzstyle{vertex}=[circle, draw, inner sep=0pt, minimum size=18pt] 
\newcommand{\vertex}{\node[vertex]} 
\begin{document} 
\[
\begin{tikzpicture}[scale=1,very thick] 
\draw[step=1.5cm,very thick] (.15,.15) grid (4.35,4.35);
\vertex[fill] (a) at (2.25,.75) [] {};
\vertex[fill] (a) at (.75,2.25) [] {};
\vertex[fill] (a) at (3.75,2.25) [] {};
\vertex[fill] (a) at (2.25,3.75) [] {};

\draw [>=stealth,->] (2.25,.75)-- (2.25,2.05); 
\draw [>=stealth,->](.75,2.25)-- (2.05,2.25); 
\draw [>=stealth,->](3.75,2.25)-- (2.35,2.25); 
\draw [>=stealth,->](2.25,3.75)-- (2.25,2.35);
\draw[very thick,->](5,2.225)--(7,2.25){};
\node at (6,2.5) {\LARGE $\mu$};
\end{tikzpicture} 
\hspace{.5cm} 
\begin{tikzpicture}[scale=1,very thick] 
\draw[step=1.5cm,very thick] (.15,.15) grid (4.35,4.35); 
\vertex[fill] (a) at (2.25,.75) [] {};
\vertex[fill] (a) at (.75,2.25) [] {};
\vertex[fill] (a) at (3.75,2.25) [] {};
\vertex[fill] (a) at (2.25,3.75) [] {};


\end{tikzpicture} 

\] 



\end{document}