On December 4th 1992, President of Mongolia, Punsalmaagiin Ochirbat declared that Mongolia is becoming Nuclear weapon free zone at the United Nations General Assembly session.
Mongolia is a party to Nuclear Non-proliferation Treaty (NPT) of 1969. In short, this treaty acknowledges only 5 countries to be nuclear-weapon states. And prohibits other countries to possess nuclear weapons. As a member of the the treaty Mongolia pledged to posses and transfer nuclear explosive devices. So, declaring itself nuclear-weapon-free zone was in a sense, logical continuation of obligations received under this treaty.
The other thing affected Mongolia’s decision was: Mongolia has two neighbors, namely: Russia and China, both official nuclear weapon state, acknowledged by NPT. Also, it has bitter history of brush with nuclear confrontation. As Sino – Soviet Union relationship went sour in 1960s, in 1969, Soviet Union concentrated large amount of military personnel near Chinese border in Mongolia. It’s later been revealed that USSR was planning nuclear strike on China if relationship further deteriorated.