\documentclass[12pt]{book}
\usepackage{graphicx}
\usepackage[font={small},labelfont={bf},labelsep= space,labelformat=default,format=plain]{caption}
\usepackage{subfigure }
\usepackage{xepersian}
\settextfont{XB Niloofar}
\setlatintextfont[ExternalLocation,BoldFont={lmroman10-bold},BoldItalicFont={lmroman10-bolditalic},ItalicFont={lmroman10-italic}]{lmroman10-regular}
\begin{document}
\chapter{مقدمه}
\begin{figure}[h!]
\centering 
\subfigure[{}]{\label{fig:figure:im1} %% label for first subfigure
\includegraphics[width=.4\textwidth]{pic3.jpg}
}
\hspace{8mm} 
\subfigure[{}]{\label{fig:figure:im2} %% label for second subfigure
\includegraphics[width=.4\textwidth]{pic4.jpg}
}
\caption{ نمودار (الف) $ f(x) $ و (ب) $ f^{\ast} $ }
\label{pic2.1} %%label for entire figure
\end{figure}
\end{document}