Lukas_Sidla said:
;D I didn't mantion any century. I want that flag because this flag is awsome
And yeah there wasn't no flag but Coat-of-arms was. It was King Mindaugas coat-of-arms (http://www.golfweb.lt/wp-content/uploads/2010/07/Karaliaus_Mindaugo_Herbas-0.jpg) And yes I'm Lithuanian and I know that things because I love my history
Sadly, there's no historical sources for coat of arms of any of the dukes from Lithuania till XIV century or so. The one you posted is not Mindaugas coat of arm.
Mindaugas supposedly did have his king seal:
but it might be a forgery as well.