\usepackage{amsthm,amssymb,amsmath} 
\documentclass{article} 
\usepackage{xepersian} 
\settextfont[Scale=1.1]{XB Niloofar}
\begin{document}
\begin{align*} 
&\ \ \Leftrightarrow\ \ \ \ \mathop {\left[ x \right]}\nolimits_\theta \cap A \ne \emptyset\vee \mathop {\left[ x \right]}\nolimits_\theta \cap B \ne \emptyset \\
&\ \ \Leftrightarrow\ \ x \in \bar \theta (A)
\mbox{یا}
x \in \bar \theta (B) \\ 
&\ \ \Leftrightarrow\ \ x \in \bar \theta (A) \cup \bar \theta (B) \\ 
\end{align*}
\end{document}