\documentclass{report}
\usepackage{titlesec}
\titleformat{\chapter}
{\normalfont\LARGE\bfseries}{\thechapter}{1em}{}
\titlespacing*{\chapter}{0pt}{3.5ex plus 1ex minus .2ex}{2.3ex plus .2ex}
\begin{document}
\chapter{Introduction}
\section{Section}
\end{document}
http://tex.stackexchange.com/questions/10326/how-to-set-the-chapter-style-in-report-class
