please click on Home page then Contact page and you'll see that the my site slightly shifts to left.

I don't understand why this happens as I created all the pages by using the 'Add page' menu. (Only difference is that the Contact page contains a form.)

do you know why this happens? and how is it possible to fix this?

thanks

I tried it but couldn't see any movement to the left.

I have a feeling that you may have seen this effect:
http://www.websemantics.co.uk/tutorials/useful_css_snippets/#jump

If that's the case, then this would fix it:

html {height:100%}
body {min-height:101%}

That would go into the CSS.

spot on!!! (i'm using Opera.) it's now fixed. thank you!

Glad you got it done. I think I should add that little fix into the default layouts...

Ok, default layouts are updated.