\documentclass[a4paper,12pt]{report}
\usepackage{graphicx}
\usepackage{caption}          
\usepackage{xepersian}
\settextfont[Scale=1.2]{B Nazanin}
\setlatintextfont[Scale=1]{Times New Roman}
\setdigitfont  [Scale=1]{Persian Modern}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\makeatletter
\long\def\@makecaption#1#2{%
  \vskip\abovecaptionskip
  \sbox\@tempboxa{#1: #2}%
  \ifdim \wd\@tempboxa >\hsize
    \if@RTL\beginR\fi\linespread{1.8}\selectfont#1: #2\if@RTL\endR\fi\par
  \else
    \global \@minipagefalse
    \hb@xt@\hsize{\if@RTL\beginR\fi\hfil\box\@tempboxa\hfil\if@RTL\endR\fi}%
  \fi
  \vskip\belowcaptionskip}
  %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{document}
\begin{figure}[ht]
\caption*{
مقدار گپ و مقادیر تجربی }
\centering
\includegraphics[scale=0.4]{61}
\end{figure}
\end{document}