I am developing an Android app.
I have a custom button with a .9.png as background, it is perfect in the UI tool in Eclipse, shown here:

but on phone it’s like this:

As you can see this is very jaggy(not sure I use the correct word)
and this is my 9-patch file:

I know I can replace this simple button with an Shape XML drawable, but does anyone know what may be causing this problem and how to solve it?
Try putting your resource in
res/drawable-nodpi/, or possiblyres/drawable/, instead of wherever it is, and see if that helps. As @MisterSquonk suggests, I think this is Android applying automatic density scaling.