This article explores the scope and nature of diplomatic immunity ratione materiae under the Vienna Convention on Diplomatic Relations (VCDR) by comparing this immunity with state immunity and immunity ratione materiae of ordinary state officials in general international law. It is argued that diplomatic immunity ratione materiae is distinct from immunity ratione materiae of ordinary state officials because ‘functions’ of a mission member should not be treated as ‘state functions’ in general but should be understood within the framework of Article 3(1) of the VCDR, which sets out the functions of a diplomatic mission as a whole. This means that the immunity cannot be upheld for serious violation of international law. On the other hand, diplomatic immunity ratione materiae is also different from state immunity both in scope and in nature. Therefore, the immunity must be understood as a unique concept which includes both the substantive issue of non-personal-liability and the procedural issue of immunity from jurisdiction. This hybrid nature of diplomatic immunity ratione materiae is the corollary of the functional emphasis of the Vienna Convention.