I used book class to write my thesis as follows
\documentclass [12pt,oneside,final]{book}
The title of some chapters are long so it reaches the page number as shown in picture.
how I can breaks Long titles of the chapter in the headers into 2 lines so it will look better
any idea

fancyhdrsuggestion would be to alter the headers/footers in another way e.g. use a smaller font, put the page number in the footer etc. – cfr Feb 19 '14 at 02:53