\documentclass{book}
\usepackage{xepersian}
\settextfont{Yas}
\defpersianfont\chapterfont[Scale=4]{XB Niloofar}

\makeatletter
\def\@makechapterhead#1{%
  \vspace*{50\p@}%
  {\parindent \z@ \centering\normalfont
    \ifnum \c@secnumdepth >\m@ne
      \if@mainmatter
        \Huge\bfseries {\chapterfont \@chapapp}\space \thechapter
        % \Huge\bfseries {\chapterfont \@chapapp\space \thechapter}
        \par\nobreak
        \vskip 20\p@
      \fi
    \fi
    \interlinepenalty\@M
    \Huge \bfseries{\chapterfont #1}\par\nobreak
    \vskip 40\p@
  }}
\def\@makeschapterhead#1{%
  \vspace*{50\p@}%
  {\parindent \z@ \centering
    \normalfont
    \interlinepenalty\@M
    \Huge \bfseries   #1\par\nobreak
    \vskip 40\p@
  }}
\makeatother



\begin{document}
\chapter{آزمایش}
\end{document}