I have customized the default chapter style like below. The chapter number font is Garamond number font, whereas the main text is Times font.

The codes:
\documentclass[a4paper, 12pt]{book}
\usepackage[T1]{fontenc}
\usepackage{newtxtext, newtxmath}
\usepackage{color}
\definecolor{RoyalRed}{RGB}{157,16, 45}
\renewcommand{\sfdefault}{mdugm} %Garamond
\usepackage[ ]{titlesec} %
\titleformat{\chapter}[display]
{ \normalsize \huge \color{black}}
{\flushright \normalsize \color{RoyalRed} \MakeUppercase { \chaptertitlename } \hspace{1 ex} { \fontsize{60}{60}\selectfont \color{RoyalRed} \sffamily \thechapter }} {10 pt}{\huge}
\begin{document}
\chapter{Introduction}
...
bla bla
...
\chapter{State of the art}
...
bla bla
...
\end{document}
So how to set the chapter title state of art as bold font?

\sfdefaulttomdugmhas very little sense. And it has little sense having Times as main font and Garamond for the chapter numbers as well. – egreg Jan 21 '13 at 16:21Garamondnumber fonts to The Times' :) – KOF Jan 21 '13 at 16:29