From 2010 until 2015, she was an associate legal officer at the United Nations International Criminal Tribunal for the former Yugoslavia in The Hague on the case of the Prosecutor v. Radovan Karadžić. Sun has previously worked with the Extraordinary Chambers in the Courts of Cambodia (the Khmer Rouge Tribunal) and the European Center for Constitutional and Human Rights. Her specialties include litigating international crimes (genocide, crimes against humanity, war crimes), international humanitarian law, and human rights law.