restore default Font

Sure, just reinstall your OS.

Tell us exactly what your problem is and someone might be able to suggest an easier way to fix it.

Yes.

First, empty the /Library/Fonts/ folder and the Fonts folder in your user account. Leave the /System/Library/Fonts/ folder alone!

Then, follow the directions in the last section of Font Management in OS X to see how to restore just the fonts from your OS X installation disk.

The link, or one of the links above directs you to my personal web site. While the information is free, it does ask for a contribution. As such, I am required by Apple's rules for these discussions to include the following disclaimer.
I may receive some form of compensation, financial or otherwise, from my recommendation or link.

when I have restore the fonts to the default, how can I recover the custom fonts? (fonts that installed by Office 2008 & Adobe products)

That's a bit different than the initial question, which was to "restore fonts to default". Which I think most people would read as you only want the original OS X supplied fonts on your system.

If you placed the fonts you cleared out of the two folders noted above into another folder, you can just drag and drop the non OS X fonts back into the /Library/Fonts/ folder. If you'd rather have pristine copies of the fonts you mention, you can grab them from the original disks.

For the Adobe fonts, I'm looking on the CS4 Design Premium disks, though you should be able to find them pretty much the same way with any of the CS applications. Insert disk 1 (that's where mine were). Double click the desktop icon of the DVD. Double click the application icon, then the folder "payloads". That folder may be on a different disk, depending on what Adobe software you have, but if you find "payloads" on that disk, the next folder to open is "AdobeFontsAll". In that folder, double click the disk image "AdobeFontsAll.dmg" to open it. Now open the folder "Assets" and then "contents". Drag and drop all of the fonts from the disk image "contents" folder to the /Library/Fonts/ folder of the hard drive.

For Office 2008, insert the installation disk and run the installer. When you get to the Installation point, click on the Customize button. Uncheck "Microsoft Office 2008" and "Dock Icons", so only "Office Fonts" is checked. Click the Install button.

i want to restore the fonts to the default because I have installed another custom fonts(graffiti fonts). unfortunately, when I checked at the Font Book, that font have replaced the Arial font, Times New Roman & other system fonts - causing when I open a pdf or check my email, what I got is message written in the graffiti fonts, I hardly can read it

Why don't you just get rid of the graffiti fonts? Junk font substitution like this is quite common, and the normal remedy is just to find them and trash them.

If you have deleted the fonts but still see incorrect fonts in your applications, you need to clear the font cache data. One way is to use Safe Mode (restart and hold down the Shift key), and then restart normally.

just, start in safe mode, then restart?

Yes. That will clear all cache data from your user account. It will also reset Font Book, so any custom font sets you've created will be gone, and all fonts in any of the various Fonts folders will be active, regardless of their state beforehand.

If you'd like to clear the font cache data without losing your Font Book settings, download OnyX. Close all running apps. Launch OnyX and click on the Cleaning heading, then the Fonts tab. Turn the check boxes on for all that apply to you and click Execute. Make sure to let the system restart.