\documentclass[a4paper,12pt]{book}
\usepackage{xepersian}
\settextfont{XB Kayhan}
\setdigitfont{XB Kayhan}
\defpersianfont\nastaligh[Scale=1.3]{IranNastaliq}
\makeatletter
\newcommand\mycustomfont[1]{\def\@mycustomfont{#1}}
\newcommand\mycustomraggedright{%
	\if@RTL\raggedleft%
		\else\raggedright%
							\fi}
\def\@part[#1]#2{%
    \ifnum \c@secnumdepth >-2\relax
      \refstepcounter{part}%
      \addcontentsline{toc}{part}{\thepart\hspace{1em}#1}%
    \else
      \addcontentsline{toc}{part}{#1}%
    \fi
    \markboth{}{}%
    {\centering
     \interlinepenalty \@M
     \@mycustomfont
     \ifnum \c@secnumdepth >-2\relax
       \huge\bfseries \partname\nobreakspace\thepart
       \par
       \vskip 20\p@
     \fi
     \Huge \bfseries #2\par}%
    \@endpart}
\def\@makechapterhead#1{%
  \vspace*{50\p@}%
  {\parindent \z@ \mycustomraggedright \@mycustomfont
    \ifnum \c@secnumdepth >\m@ne
      \if@mainmatter
        \huge\bfseries \@chapapp\space \thechapter
        \par\nobreak
        \vskip 20\p@
      \fi
    \fi
    \interlinepenalty\@M
    \Huge \bfseries #1\par\nobreak
    \vskip 40\p@
  }}
  \renewcommand\section{\@startsection {section}{1}{\z@}%
                                   {-3.5ex \@plus -1ex \@minus -.2ex}%
                                   {2.3ex \@plus.2ex}%
                                   {\@mycustomfont\Large\bfseries}}
\renewcommand\subsection{\@startsection{subsection}{2}{\z@}%
                                     {-3.25ex\@plus -1ex \@minus -.2ex}%
                                     {1.5ex \@plus .2ex}%
                                     {\@mycustomfont\large\bfseries}}
\renewcommand\subsubsection{\@startsection{subsubsection}{3}{\z@}%
                                     {-3.25ex\@plus -1ex \@minus -.2ex}%
                                     {1.5ex \@plus .2ex}%
                                     {\@mycustomfont\normalsize\bfseries}}
\makeatother
\mycustomfont{\nastaligh}
\begin{document}
\tableofcontents
\part{مقدمات و تاریخچه}
\chapter{مقدمه}
\section{پیدایش مسئله}
\subsection{از کجا شروع شد}
\subsubsection{کی شروع کرد}
\newpage
این یک صفحهٔ جدید است.
\end{document}