Okay, I know that the question is hard to digest.
So I hope this attached picture will make more sense of my question.
Basically I want to maintain some space between my header and my tablix after first page.
I use bottom border for my header and user don't want to see the table and the header just glued together.
tried to put empty textbox (hidden) but not really working as I can't repeat the textbox on subsequent page.
Mark Bannister's answer is correct. Your pictures incorrectly place space between the header and content - tablix. The header shares a border the content - tablix not the body.
The solution, is to place an empty header in your report and that will create the space between the top of the page and the content - tablix you need.
It looks as though you have a gap between the top of the Report Body section and the top of your tablix - this would lead to an extra gap on the first page, which is not repeated on subsequent pages.
If you move your tablix so that there is no gap between it and the top of the Report Body section, this should resolve the problem.