After searching for 2h. I finally found the answer..
here:
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Four things are required for transparent icon backgrounds:
1) 'System Properties' -> 'Advanced' tab -> Performance
'Settings' ->
'Visual Effects' tab:
"use drop shadows for icon labels..." must be checked
2) Right-click on Desktop -> 'Arrange Icons by' -> "Lock Web
items..." must
be CLEARED
3) If you have any web content on your desktop, transparency won't
work.
To check for web content: Display Properties > Desktop tab >
Customize
Desktop button > Web tab > make sure all checkboxes are
clear.
4) Wallpaper must be an image file, not html.
credit to
http://www.d-a-l.com/help/showthread.php?t=35272 and every
friendly people who reply in this topic.