thanks for the respond but unfortunately it's not solved the problem, the Border on DatepickerTextBox refer to the black border around it.
below i provide the screenshot if i set it like this:
u can see that the white line is still there x___x
owh yeah, i forgot to remove the property HorizontalContentAlignment="Stretch", if i remove this and make the field empty, then it will be like this:
as u can see, i think the whit line has nothing to do with border, CMIIW