Do I ( Sri Lankan National ) need to obtain visa while only transiting at HK International Airport?

Yes, you need the entry permit for transiting in Hong Kong. You have to apply for it before your departure, the documents needed for the application are: application form (ID 1003A), your recent photograph (affixed on page 1 of the application form ID 1003A), a photocopy of your passport, a photocopy of proof of your financial standing, e.g. bank statements, savings accounts passbooks, tax receipts, etc. and proof of employment (if any), e.g. company leave letter, salary slips, etc., and a photocopy of your onward flight ticket.